Duplicate contacts creating at random

Hi,

We have a situation where, after the 365 upgrade, the system started creating duplicate contacts. According to the system the contacts are created by Users but the most likely scenario is that the system somehow created them. The reason being that the contacts are created by the multiple. Each day there are approximately 5k new duplicate contacts...

We've set up a duplicate detection rule which should take care of new duplicates but I checked this morning and the system keeps on creating them. Could it be through Outlook? 

Would appreciate any help we can get.

Best,

Anders