if you have a FilePicker profile set with an empty (optional) Top Directory, a
prefix of '/' is set the second time the FilePicker is used.
If you don't have any FilePicker profiles set the prefix is always empty '' and
works correctly.

To reproduce the error:
- create a 'Default' FilePicker profile with an empty 'Top Directory'
- use the MicroTiny WYSIWYG, click, 'insert/edit image', select an image, then
select another image.
- a double prefix '//' shows after uploads in the selected url
- remove the 'Default' FilePicker profile and it stops happening

Note:
/modules/FilePicker/lib/js/cmsms_filebrowser/filebrowser.js, line 26
		if( settings.prefix ) file = settings.prefix + file;

I'll investigate further where settings.prefix is set an what else it is used
by...