QGIS python plugin appends values every time it's started

I've written a small python plugin (using plugin builder as base) to simplify the expression process for a given layer (just a few comboboxes that are used to create a query). The combo box populating is done in the def run(self): part with a simple .addItem loop. However when I close the plugin and reopen it again, the values are added again. So if it's "a, b, c" and I close/open the plugin again, it's "a, b, c, a, b, c" in the box.. can I stop/avoid that? Or should I just wrap it in a case that sets a flag on the first run?