Is saying "candor and honesty" grammatically correct?

Maybe I'm splitting hairs a bit, but if you use

• candor to express impartiality and a freedom from bias, and
• honesty to denote a fairness and straightforwardness of conduct

then candor and honesty reinforce each other in praise of the probity of the person being thanked.

"Candor" means frankness or openness, "honesty" means truthfullness.

As Dusty notes, you can certainly be honest without being candid. If you are reluctant to tell the truth and it has to be pulled out of you, or if you beat around the bush or use euphemisms, for example.

Whether you can be candid without being honest gets into the exact definitions, or maybe connotations, of the words. As I understand them, if you said, "I was really mad because Bill insulted me," that would be "candid": you are freely and openly expressing your opinions and feelings. But if Bill's insult was trivial, or if it was prompted by some bad action on your part that might lead an outside observer to say that you were to blame, then your statement is not really honest. So in that sense I think you could be candid without being honest. It's a fine line, though.

In any case, sure, I try to avoid being redundant or repetitious or saying the same thing twice. But redundancy is not necessarily a bad thing. It is often used for emphasis. If you say, "Sally is an incredibly beautiful, stunning, gorgeous girl", you might say that technically this is the same as saying, "Sally is pretty", just redundant. But clearly the first sentence is more emphatic than the second, precisely because it is redundant.

Despite how ancient this question is, I have a vivid example to explain the difference between "candor" and "honesty", and how you can be candid without being honest, or vice versa.

Redundancy and Grammar

First things first: as Dusty correctly notes, the sample sentence would not, strictly speaking, be ungrammatical if it was redundant. (Although contra Dusty, redundancy can be ungrammatical: in the sentence, "He took a bite of the the apple," the repeated "the" towards the end is both.) The concern, insofar as there is one, is stylistic rather than syntactic. So: if we say,

Thank you for your candor and honesty

have we qualified ourselves as Honorary Right Honorable Chairperson of the Needlessly Repetitious Department of Needless Repetition?

Truthful Tone vs. Truthful Content

The answer is "no", because the two concepts focus on different aspects of "truthfulness" (or maybe, for reasons that will become clear, we should say "truthiness" instead). The difference boils down to: Candor is about tone; honesty is about content. And as we shall see, it's perfectly possible for a liar to have one but not both.

Honesty Without Candor

Consider the example of a BOGO deal for a mediocre amusement park. The deal is advertised on posters and those little promotional postcards that clutter up your mail as follows, in large print:

BIG GIVEAWAY SWEEPSTAKES! WIN A TRIP TO AN AMUSEMENT PARK COMPLETELY FREE!*

and then, if you flip over the card to see what the asterisk says, in tiny print:

*Terms and conditions apply. Call 1-800-PLAYFUN for details.

If you call 1-800-PLAYFUN, you will be informed that you are "lucky caller number ____." (In fact, if you call them back, you will always be informed this, because everyone who calls wins; technically, a lottery where no one ever loses can still be considered a lottery, albeit a trivial one.) The person on the other end asks if you are at a computer, and walks you through a long process in which you are mined for a vast amount of consumer data such as birth date, location, email address, daytime phone number, and your preferences regarding a number of common household items. Having done this, you are then informed that to collect your free trip to the amusement park, you are required--as part of the "terms and conditions"--to enlist a friend or family member to go with you, who will of course pay the full price of $375. Which probably means you and your relative or acquaintance will end up going halfsies on the one ticket to get the other one free.

"So it's not really 'completely free'," you will say, and the person on the other end will explain to you that your ticket is completely free, it's the other one that isn't. They will strenuously argue with you that this still counts as getting a "completely free ticket", and while they are technically correct (the best kind of correct?) you will be infuriated at the fact that you went to all that effort only to find that the ticket was "COMPLETELY FREE"...with regards to a lawyerly definition of both "completely" and "free".

Notice that at no point did the advertisement or the phone worker tell you anything inarguably false. So in that sense, they were honest with you. But they went to great lengths to obscure some extremely relevant, and unattractive, details about the deal for as long as possible. In other words, they were clearly not candid.

Candor Without Honesty

To see how we can have candor without honesty, let's consider a different approach to salesmanship for the same buy one, get one free "sweepstakes" deal to the same mediocre amusement park. In this scenario, when you call in, the person on the other end acknowledges the elephant in the room straight away. They admit--freely and within the first 90 seconds of the call--that, actually, this is a "sweepstakes" where everyone wins; and yeah, you do have to go with a friend/family member who pays full price of $375 in order to access your "completely free" ticket.

But!

The rides are incredible; the lines are short; the park's restaurants are Michelin-star quality, yet affordable; the scenery is gorgeous; the amusement park is "not too far" from a bustling downtown with great nightlife; and oh yeah, did they mention how the park is well-attended by sexy people from the nearby university?

But if you and your friend/family member are sufficiently dazzled by this pitch, and disarmed by the salesperson's actually copping to the true nature and expense of the "sweepstakes", to purchase the ticket and go, you soon learn that the rides are pathetic; the food is sub-high-school-cafeteria level, and exorbitantly priced (e.g. $8 for a tiny basket of soggy, cold fries); the lines for both are endless, and there's no shade anywhere; and the park is 15 miles from the nearest town, and surrounded by empty desert, ugly asphalt, and (occasionally) gas stations, without even a mountain range nearby. The beach is 30 miles away. And needless to say, those hot coeds are nowhere to be seen; most of the attendees seem to have been bused in on trips from senior centers in the tri-state area (remember, the rides are pathetic).

Unlike the mealymouthed salesperson in the first example, no one could possibly describe the salesperson in this example as honest, since they egregiously lied to you about pretty nearly every relevant aspect of the amusement park. Yet they were indeed candid with you--at least selectively, with regards to the "sweepstakes" and the BOGO deal. That candor was integral to their ploy: seasoning an audacious farrago of falsehoods with a dash of refreshing truth up front, and maintaining the same confidential and direct attitude throughout your interaction, in order that you would think they were being equally truthful when it came to the miserable park experience itself.

The distinction in real life

While a lot of the time in colloquial settings, "honesty" and "candor" are used interchangeably, the two words may have distinct meanings in professional contexts. One example that was in the news in recent years (long after the question was asked) was the firing of Andrew McCabe from his post as deputy director of the FBI. McCabe's firing was widely perceived as an attempt by then-President Trump to scapegoat "wrongdoers" and "liars" at the top of the FBI as an explanation for why Trump was being investigated over his numerous and suspicious Russia connections. However, it was officially justified as being due to "lack of candor" on McCabe's part. Notably, per the linked Atlantic article:

“Lack of candor is untruthfulness or an attempt to dissemble from the point of view of the investigator,” said Dave Gomez, a former FBI agent and a senior fellow at George Washington University’s Center for Cyber and Homeland Security. “The problem comes when, in answering a question, the person under investigation attempts to spin his answer in order to present his actions in the best possible light. This is normal human behavior, but can be interpreted as a lack of candor by the investigator.”

According to former FBI officials, the obligation of bureau employees—agents or otherwise—to have candor in performance of their duties comes from the need for FBI officials to represent the federal government in legal proceedings, act as witnesses in trials, and manage informants. Under the FBI’s standard, candor is not simply telling the truth—it confers an obligation to disclose relevant information even if an investigator has not directly asked about it."

The article then gives instances of "lack of candor" firings, some of which involve literal dishonesty and others which involve not outright falsehood but equivocal casuistry. It goes on to note that this is different from the standard needed to prove "perjury" in criminal trials, which does require knowingly telling a falsehood (dishonesty), not just a statement which is true but misleading (lack of candor). So not only are "honesty" and "candor" different--real people's livelihoods, or even their liberty, are and have been at stake over the distinction.