Find word by given characters

[Bischoop]

I'm working on a script i which user inputs letters and then a printed
words containing those letters. The scripts works however I can't solve
one problem , it prints also words in which these letters occur more 
than once.
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Fore example:
Letters: at
Output: auto, autobahn.
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I supposed to not print word: "autobahn" because I've given it only one
letter "a".

Here is a link to the code:
https://bpa.st/UTAA

Thanks in advance