So this is a problem I've noticed happening fairly frequently, and I get it, most people are still pretty new and Blight Rounds don't really have their mechanics fully explained in-game anywhere either. I did the same thing when I started playing Ranger back on JP. But anyway...
In general, each enemy can only have ONE part under the effect of Blight Rounds, with a few exceptions. And that one part can only be affected by one bullet at a time. So if there are two Rangers in an area, and one marks the head of an enemy while the other marks a weak point on an arm or something, that first Ranger's bullet is completely wasted. One bullet isn't a huge deal, but if both Rangers then continue trying to put their mark back, that's 11 bullets that wind up being wasted, and much more downtime before Blight Rounds can be reapplied after the last one expires. So please, if you see a Blight Rounds marker on an enemy, don't shoot your own until it expires!
That said, sometimes people miss and hit the main body, or better weak points become exposed, in which case it may be appropriate to overwrite a previous marker. For example, if there's a Blight Rounds marker on Luther's beak, then his clock or one of his shoulders opens up, you should go ahead and move it to the new spot to get as much DPS out of that window as possible.
And that's all I've got! Thanks for reading, and go kill some Falspawn!