How do you concisely make clear something exists only once?

I am struggling to find an appropriate and convenient wording to describe that something exists only one time in the whole world. Usually the word “unique” implies exactly that, but it’s been misused for things which are not really unique that it lost its power for this purpose imho.

As an example here are some sentences about a painting and why I don’t think the phrase is good:

• This is a unique painting.
  • It feels like there could be still a copy of the painting out there and unique relates to the feeling / atmosphere / other feature or to the unique kind of the painting. Also, as I mentioned before, unique doesn’t carry over enough power to the uniqueness.
• This painting has been painted only once.
  • This is misleading in the way that you might understand the process of drawing the painting has been done only once. Also has a bad sound to it.
• This painting is a single copy.
  • Maybe it’s just me, but having the word “copy” to describe something exists only once seems contradictory.
• This painting has been manufactured only once.
  • Sounds very cumbersome and not fitting to anything that’s not been literally manufactured. Would rather fit a plaster figure more than anything else.
• This painting is a unicum
  • This might be it, but I’ve never ever heard anyone actually say this, so I am afraid this might throw people off and a non-native speaker easily might not know this word.
• This painting exists only one time in the world. / This painting does not exist a second time on the world.
  • This is kind of what I am looking for in terms of the message, but both versions sound very cumbersome and are too long.

I am looking for a very short expression or just one word which doesn’t sound cumbersome, doesn’t leave room for interpretation and is well known