I was thinking about this recently, couldn't you alter one word in message to create two copies, and then basically perform binary search until you find the person?
> ... first draft of the report, I came up with an idea to make each one unique."
> "They've been doing that for years," Holmes noted. "All one must do is misplace a comma here and there. Easiest thing in the world. If the newspeople are foolish enough to print a photograph of the document, we can identify the leak."
> "Yes, sir, and the reporters who publish the leaks know that, too. They've learned not to show photographs of the documents they get from their sources, haven't they?" Ryan answered. "What I came up with was a new twist on that. Agents and Agencies has four sections. Each section has a summary paragraph. Each of those is written in a fairly dramatic fashion."
> "Yes, I noticed that," Charleston said. "Didn't read like a CIA document at all. More like one of ours. We use people to write our reports, you see, not computers. Do go on."
> "Each summary paragraph has six different versions, and the mixture of those paragraphs is unique to each numbered copy of the paper. There are over a thousand possible permutations, but only ninety-six numbered copies of the actual document. The reason the summary paragraphs are so -- well, lurid, I guess -- is to entice a reporter to quote them verbatim in the public media. If he quotes something from two or three of those paragraphs, we know which copy he saw and, therefore, who leaked it. They've got an even more refined version of the trap working now. You can do it by computer. You use a thesaurus program to shuffle through synonyms, and you can make every copy of the document totally unique."
> "... Say we send this memo to the same people who received the other memo — well, we don’t have to include anyone in this room — you’re in on it — but we’ll send copies to every one of the others. Each copy of the new memo would be exactly the same, except for one word. In each memo there would be one word not in the others. We would keep a record of the person to whom we sent the memo — and beside his name we would jot the unique word that was in his memo. Do you see what I’m driving at? When the memo goes out, the person here who is betraying us will pass it on word for word to de Vroome, isn’t that so? Your informer in de Vroome’s headquarters would learn of it and report back to you. Since no memorandum would be precisely the same as the others, because of the single word change, you’d look for the different word in the memo de Vroome received and be able to find out the person who had passed on his copy of the memo. You'd know your traitor."
(back when forums were still a thing)