I can't find dictionary entries nor corpus examples for unsingular, and the few examples in books.google point to it being an antonym of singular.
On the contrary, indisimulable doesn't point to being one of a kind or, on the contrary, a run of the mill item, but to the fact that you wouldn't be able to conceal the indisimulable thing no matter you used the better techniques and efforts, even if you counted with the willing complicity of the observers: the indisimulable thing will continue to stick out like a sore thumb, and the indisimulable feeling will be shown clearly. An indisimulable thing is an elephant in the room.
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