Very strange.
#1) Please enable the debug logging feature so we can see if there is any clues in the log file:
Using the Debug Log to Solve Transaction Related Issues on a Site
#2) Next, do the following test also. To make sure there isn’t another plugin that is causing the request to be submitted multiple times on this site:
Test for Plugin and Theme Conflict Before Posting an Issue or a Bug
#3) Then do a test transaction and let me know what you see.