Glimmering Mark. Stackable?

Can multiple people cast Glimmering Mark on a target?

As in, can 3 people cast it on a target to do 63 dmg/sec? 630dmg/10seconds?
Or will the hex overwrite itself?

like all hexes, it overwrites.

All hexes overwrite, to a degree. Hexes like parasitic bond will still provide the healing to their respective casters when each was supposed to end, even if they are "stacked." Things like this are important to keep in mind.

except asuran scan

Same hexes stack, but only on the effects given back to the casters. Any offensive effect to the target doesn't stack.

I don't know if it's true for all hexes, but it should be for most.
For example, Insidious Parasite would only damage the opponent once, but it would heal all casters of that hex (untested). And Asuran Scan would increase damage for all casters, not just one of them (tested).

I believe overwriting is the wrong term to use altogether. It is possible for a target to have more than one of a hex at a time, it's just that they will only be effected by the most powerful. So if Monk 1 was to cast a +8 pip Healing Breeze on Ranger 1, then Monk 2 casts a +7 HB on the same Ranger 3 seconds later the Ranger would get +8 regen until the last 3 seconds at which point he will get +7 from the second cast. This is why things that benefit multiple parties (i.e, AScan, Parasitic Bond, etc) can stack in a way.

Will a less powerful GM "overwrite" a less powerful one?
I would test it, but I don't have access to the game at the moment.

Test on the Isle of the Nameless - bring a low ranked Glimm Mark on your bar and a hero with a higher ranked GM.
Micro the hero's copy onto a practice target and then cast it yourself. If you see numbers, it overwrote it.
Test stackability with Arcane Echo if you're not convinced there.

Tested. Works as I explained above. The foe has both hexes but only the most powerful takes affect. If the low damage one is cast by the player first, numbers will stop appearing when the hero casts the higher level one. If the hero casts first, numbers from the players one will not appear until the higher level one has expired.

Ah yes, missed your post.

Well that blows. So only the more powerful spell is doing damage? If 3 people use GM with 12 air magic at the same time. Two of them, in a sense, is wasted?

Well yeah.
But why would you want to waste your elite on Glimmering Mark anyway? Even if it did stack like you want it would be pathetic.