How did Zamenhof prevent people from...
How did Zamenhof prevent people from claiming Esperanto as their own language and not something he invented?
Is there any account of anyone else claiming to be Doktoro Esperanto? Or claiming that it was a non-invented language?
I think your question presumes there was such a claim. (I looked, albeit briefly, just now. My search has not yet found any such claim within a relevant timeframe.) If there was, it is likely that a written account of the proof of identity exists somewhere which would answer your question (and if anyone knows of such an account online and wants to link to it that would be great). If there wasn't, then the answer to your question is simply that no-one else claimed to be the inventor of the pseudonymously published pamphlet.
Your question also allows for the ambiguous interpretation that "people ... claiming Esperanto as their own language" might mean that Zamenhof would have attempted to restrain a person or group from deciding that henceforth Esperanto would be their language. Wikipedia states:
In the book Zamenhof declared, "an international language, like a national one, is common property" and renounced all rights to the language, effectively putting it into the public domain. https://en.wikipedia.org/wiki/Unua_Libro - in such a context I don't think he would have stopped them.
Ido itself, and other Esperantidos since then, came out of a group of people attempting to reform Esperanto (as it were, another way of claiming Esperanto as their own). Zamenhof did not prevent this (which is not to say he did not sometimes try). Instead history makes it look as if any prevention of languages created by people splitting off from Esperanto was largely the result of either the unfortunate deaths of advocates of reform (such as https://en.wikipedia.org/wiki/Louis_Couturat ), of increasing dilution of each project by each new project (such as https://en.wikipedia.org/wiki/Novial ), or by popular "vote" (
While many of the prominent, well-educated, and practical-minded Esperantists joined the ido faction, the rest rallied around their betrayed hero. More than thirteen hundred unashamed idealists from forty countries showed up at the 1908 congress in Dresden. [Arika Okrent in, In The Land of Invented Languages]). (And as a further reference https://en.wikipedia.org/wiki/Ido_(language) )
Anyway, if such a claim was put forward, I suspect Zamenhof who had developed Esperanto for about a decade before he published the Unua Libro, would have been able to put his own claim quite convincingly. There were people who knew he was developing a language and had heard it (
On December 17, 1878, a Proto-Esperanto congress convened. Despite his shyness, Ludwik had convinced some of his schoolmates to involve themselves in his project. [Arika Okrent in, In The Land of Invented Langauges]), and there were likely (sufficient) written works on the development of Esperanto (in spite of his father burning some of them). So by 1887 when the Unua Libro was published, if anyone else had challenged his position as either author of the book or inventor of Esperanto, it is reasonable to presume Zamenhof would have had sufficient evidence to back his own claim.
There is an account that claimed to be other user who now has _true on his username, though
People knew Zamenhof was the inventor, but it wasn't something he wanted to claim. He was shy and very humble. He worked as an eye doctor for penny's on the dollar just to help fellow Jews even when they couldn't afford it. He did try to squash reforms, but he was also for reforms. If anything his reason was more he didn't mind reforms but felt reforms early in the history of Esperanto would kill it. He was well aware of the death of Volapük and how reforms and strife between the creator and others were at fault.