The Oxford English Dictionary has three definitions of "overlap" ("overlapping" is just the gerund version) depending on the context:

Definition 1

overlap: extend over so as to cover partly

An example sentence is: "The canopy overlaps the house roof at one end."

Definition 2

overlap: cover part of the same area of interest, responsibility, etc.

An example sentence is: "Their duties sometimes overlapped."

Definition 3

overlap: partly coincide in time.

An example sentence is: "Two new series overlapped."

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I don't know the rules of Clash Royale so I can't tell you exactly which definition is most relevant for your case. As others have mentioned, you could just ask somebody who knows how to play the game. Perhaps even do a little Google search. Although, it sounds as if they mean that you are not allowed having more than one card of a particular kind in your deck. Don't quote me on that since I don't know the rules of the game, though.

The short answer is yes, “no card can be used in more than one deck.” However, I recommend that you read the rest of my answer because it brings up some important concerns I had while writing my response. I strongly recommend that you edit your original question to include a screenshot of the message and any additional context that could be helpful to someone answering your question.

Your question is too ambiguous for anyone who hasn’t played Clash Royale before. However, I used to play the game, so I think I can answer your question. I just downloaded the game again to see where you found this message, but I couldn’t find it. I also did a few fairly exhaustive Google searches and didn’t find it. This means that my answer could be incorrect (depending on the context of the message). Even though I couldn’t find it, I saw that you can only make one deck for a Clan War, so I think you might have read it wrong. I will still answer the question assuming there is no error in your quoted message.

For anyone who hasn’t played the game, I will describe the relevant details in this paragraph. Two players battle each other with their decks of eight cards. Each card represents a different animals, creature, person, etc. These cards are deployed onto the battlefield and try to destroy the opponent’s towers. Those are the fundamentals of the game. Players can join groups called “clans,” in which they can participate in “clan wars,” a battle between two clans. I believe that these details should be sufficient for a basic understanding of the question.

One definition of “overlapping” is: “cover part of the same area of interest, responsibility, etc.” This is the definition that applies to this case. What the message in Clash Royale means is: “you can’t have the same card in multiple decks.” If you were to have the same card in multiple decks, they would be occupying the same “area” (being used in a deck).

A more intuitive way to approach this is to think of each card as being a physical card from a deck. If you make four discrete piles of cards, you can’t include the same card in more than one of these piles; it would be physically impossible to do so. The cards in Clash Royale are being treated in the same way.