Probably the single most dreaded error when it comes to handing network shares.
Multiple connections to a server or shared resource by the same user, using more than one user name, are not allowed. Disconnect all previous connections to the server or shared resource and try again.
Apart from the fact that this »security measure« is a complete joke, you can get the error even when using the same credentials – once implied (i.e. no /USER argument, which means it uses automatically your current account), once explicitely (i.e. /USER:%Username%, which resolves to exactly the same credentials).
Also, you get the error when using the same explicit account on two different domain controllers belonging to the same domain (i.e.: log into server A, mount a drive from A, fine, mount a drive from B, ERROR).
TL;DR: Never use “/USER:%Username%”, it will blow up in your face. Or, actually, never use “/USER” at all, since it’s unreliable in any case.
(Note that this »feature« is client-side. OSX and Linux clients will have, as usually, no problems.)