I have two python pugins, each of them imports gtk.
The first plugin (A) works fine, but if I start the second one (B) (both plugins are "gui" plugins), then a traceback appears:

Plugin A (short as possible example)

import pygtk

pygtk.require('2.0')

import gtk

import Gnumeric

def copy_from_above(gui):
    pass

fill_from_above_ui_actions = {
    'Copy_from_above':   copy_from_above
}

Similar is Plugin B

Traceback (most recent call last):

+ Trace 230191

E Execution of module "transpose" failed.

** (gnumeric:2108): CRITICAL **: gplp_unload_service_function_group: assertion `IS_GNM_PLUGIN_SERVICE_FUNCTION_GROUP (service)' failed

If I start plugin A (starts ok), then plugin B will fail.
If I restart Gnumeric, first start plugin B (works ok), then launching plugin A will fail again with the traceback message above.

Is in 1.10.17 AND in 1.11.3 (in 1.11.3 with PyGI (from gi.repository import Gtk))