Check EXTENT based iteration code for deduplication bugs

[ramSeraph]

Saw something fishy while using EXTENT based iteration using GetFeatureInfo.. The extraction was interrupted.. maybe something wrong in that codepath. Leaving an issue here to remind myself to investigate

[ramSeraph]

Ran into this again. There is definitely a problem. This time it was GetMap iteration.

[ramSeraph]

Made a change at 77c837d. let's see if it fixes it.