Yes, there's a rule for neuter adjectives with roots ending in a double consonant: when the -t ending is added, it replaces the final consonant.
glatt -> glatt (phew, no awkward triple t)
trygg -> trygt
Exceptions are made in cases where removing the double consonant would make the word identical to another neuter adjective (one whose root ends in a single consonant):
ful -> fult
full -> fullt (!)
I actually can't think of any other examples, so you needn't worry too much about running into exceptions you don't know about.