> Sadly there's a rule not letting you use a known letter more than once in a word (once A is known, you can't write ALPHA)

This is not entirely correct (or at least it's worded a bit weirdly).

If you know that A goes in a certain place you can still try a word using A more than once. It's only once you know that A does not appear twice (because you tried a word containing A twice, and the second A way gray!) then you can no longer use A twice.

In your case you tried MAMMA which contained 2 As and the second one gave you the information that there's no second A.