Is this expression against idiom?

[floratang]

at one of entrances of a building ,a sign say "please use other doors".

[Mister Micawber]

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Not an idiom, but the grammar is slightly foreshortened in sign-speak. It is asking you to use the other doors, because these doors (the ones that the sign is hung on) are out of order or otherwise not to be used.
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_________________
Canadian-American native speaker
who teaches English for a living at Mister Micawber's
ESL cafe: Interview with Mr. Micawber