Most people would use these words as synonyms today, I know I do. But they do actually have different meanings.
-Trøtt: (someone/thing is) boring, uninteresting, tedious, uncommitted
-Trett: tired, exhausted, sleepy,
But no worries, just stick to 'trøtt'. Everyone will understand that.
If most people would use those synonymously, then they do have the same meaning. Bokmålsordboka lists them with the same meaning.
'trøtt' is probably the most common variant though.
dude i am norwegian and we never use trøtt when ita boring(im testing this app