Jedit version 5.1.0 standalone, using Java 1.7.0_40.
Running Mac OS 10.9.3.
Using BufferTabs plug-in.
Maybe the "go to next buffer" command is being invoked on key-down and on key-press? Something like that? The same happens with "Goto previous buffer" -- it jumps two buffers backwards.
I am unable to reproduce this. But the keys i use are ctrl+pgup and ctrl+pgdn
Does it matter what shortcut you use?
Try Utilities - Troubleshooting - Keyboard Tester to see how things are being detected by jEdit.
What keyboard layout do you have? This might be a duplicate of #3905 or #3933.