SlackBuildsOrg/libraries/libunique3

Unique is a library for writing single instance application.
If you launch a single instance application twice, the second
instance will either just quit or will send a message to the
running instance.

Unique makes it easy to write this kind of applications, by
providing a base class, taking care of all the IPC machinery
needed to send messages to a running instance, and also handling
the startup notification side.

This is the gtk+3 version.