Duplicated brushes

Sorry if this has been posted, but what causes brushes to duplicate
over themselves sometimes? I get this problem from time to time
when im carving. For example I just carved an arch between two
walls and I was deleting the excess that players wouldnt see.
However, theres multiples of the same brush on the excess part, so I
have to select and delete several times. And I know its not a problem
and infact duplicates because I have moved them and seen all the
multiples before. Thanks for the help.

Accidental ctrl-c then ctrl-v? To be honest I've never come across this
as a problem before. Sounds to me like you are accidentally doing some
action that is causing it. An easy way for example, to do this would be
to unwittingly hold shift while moving brushes and then moving them
back, thereby cloning things. Maybe others can shed some light.

I've seen it happen a few times, but it's always been an error on my part (In the way ReNo specified, coincidentally, shift-dragging and then moving them back.)

Building brushes manually (like 4 for a wall with a hole) instead of
using the "carve" function in hammer can save you a lot of problems
here. It looks like it's much more work but in the end it's often a
faster (and cleaner) way...

I've never seen the carve tool create multiple overlapping brushes before; in any case, stop using carve for anything more complicated than rectangles!

I have seen on a couple occations where carving caused what "seemed" like brushes out of nowhere. what it really did was create invalid brushes. the offencive things stretched into infinity and became quite noticable. they were in actuality very small slivers originally, most less than one unit thick, but with the invalid nature became huge things. so yeah i have seen weird things that seem to be created from nothing.

/me has to wonder yet again, with the newest version of hammer so recently released, why did they yet again retain that carve button.?!