[pwinnski]

Generally the "rule" in English is the same as the one in German: a double consonant ensure the preceding vowel is pronounced the short way rather than the long way.

The saddest sadist, the matter of maters, and the bitter biter, for example.

[lucian1900]

That’s not long or short, but different vowels entirely. A better spelling system would express the diphthongs explicitly, or at least as diacritics.