DR for tokens
You did it too quickly, is the simple answer. DR kicks in as an ‘anti-farm’ measure, if you are completing too many dungeons too quickly, then your rewards will be diminished by half each successive run, this is most commonly observed in CoF when people are farming that.
Without knowing what dungeon you’re doing, my suggestion is to go and do an easy, fast dungeon in between each of the ‘main’ dungeon runs that you’re doing for your armour. This is a theory that I used to follow when I was farming a lot of paths, it could be complete rubbish and I was working on confirmation bias.
I don’t believe ANet ever released the specifics on the DR system. I guess you could avoid doing the dungeon too many times, too quickly. When in doubt, perhaps go have a small break between each run, or each second run. I’m sure some others have their own theories on how DR works, perhaps you’ll get some other opinions to add to mine
If it’s long enough between completing the same path on a different character you’ll still get 60. If it’s lower than whatever the threshold is, you’ll get the 30. But it should only happen if you’re repeating the same path over and over, as Cookie said. When I farm quick paths like TA Up or SE 1 for tokens I throw in a CoF 1 in between to make sure I don’t hit it. Seems to work so far.
Ya the 60 is character specific, but 30 is what 60 DR’s too first (just like your normal repeat 20’s DR to 10, it goes to 1/2). The DR is account based.