Pattern Matching Exclude Duplicate Characters

With regular expressions in the mathematical sense, it's possible, but the size of the regular expressions grows exponentially relative to the size of the alphabet, so it isn't practical.

There's a simple way with negation and backreferences.

grep '[spine]' | grep -Ev '([spine]).*\1'

The first grep selects lines that contain at least one of einps; the second grep rejects lines that contain more than one of any (e.g. allowing spinal tap and spend but not foobar or see).