Signet of Fire VS Flame Burst
Burns stack in duration. I haven’t checked this exact combination, but that is the rule for burns.
They are useful together. Since staves don’t have a lot of burning opportunity besides Flame Burst, I use Signet of Flame and Flame Burst to make sure any enemy I fight has constant burning on them.
If an enemy survives long enough to experience all the burning from both Signet of Fire and Flame Burst, I have to wonder whether we’re using the right build.