GIMP single-window-mode not start

[rich2005]

A couple of things to try

Edit -> Preferences -> Window Management

Tick Save Window Positions on Exit if it is unticked
Click on Reset Saved Window Positions to Default Values Restart Gimp and that gets 3 window mode. Now try Windows -> Single Window Mode

or

The setting is held in the file ~/.gimp-2.8/sessionrc rename that to disable it ~/.gimp-2.8/sessionrc-bak
Start Gimp up - It will be in default 3 Window mode. Now try Windows -> Single Window Mode