With a bunch of new trap skills and reworks coming with incursion, the unique jewel Unstable Payload which gives up to a 12% chance for traps to trigger again. Stacking these you can get near 100% chance to trigger traps twice, this has a huge benefit of bypassing trap limits and it costs no extra mana. Is unstable payload worth stacking? Just make sure that you have more than enough PoE orbs cheap before you start this game!
Traps that are casting (such as flamethrower trap) or re-arming count against your active traps limit, so if you go above your maximum number of traps they will actually just be replaced via killing them mid-cast. As you know, U4GM have shared a lot of guides, you can also find some useful tips from it too.
With Advanced Traps Support you will likely be able to have near 100% uptime on Flamethrower Trap anyway, since the cooldown might be 6 seconds and the duration maybe 4 seconds, but you start with three charges so by the time you have used it three times in sequence you would have recharged two, meaning you can use those and have recharged one more, then use that one while you get yet another charge.
For a final total of 20 seconds of continuous flamethrower uptime before you need a second of cooldown thrown in to get another charge. Most bosses either take less time than that or have phases where they won't be hurt anyway so you can recharge then.
Even traps and mines that are in the process of casting something can be removed due to hitting the trap or mine limit. Just like how when you cast a Spell Totem when you're at your totem limit, the oldest totem dies even if it's in the middle of casting something.
"Traps trigger twice" is basically the same thing as saying "when a trap finishes its cast for the first time, instead of killing itself, it re-arms itself at its location" so it doesn't even necessarily trigger a second time unless there's an enemy there to trigger it (although, it would now trigger at the end of its duration anyway, so that's not as big of a deal as it once was).