If there was a second of the same letter, but in the wrong place, it would be yellow. If it's gray, that means there is no repetition of that letter. So once you've seen that the second one is gray, you have the information you need to know that letter is not repeated.

Mamma has 3 M's.

Grepping /usr/share/dict/words shows 110 words that have five letters and the same letter 3 or more times, e.g. sassy, added, daddy, nanny, melee, levee, mummy