From time to time you may see a big, scary red error message on top of your Facebook Ads Manager, like this:

Turns out that this CAN be triggered by perfectly normal behavior and nothing being wrong with your pixel installation at all.
When you click through to the Events Manager as the message suggests, you may find this message there: "Redundant Purchase Event". It shows a link to "Affected URLs".

CLick that and examine the URLs - open each of them in a new tab:
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In this case, the four than-you pages that open show four separate purchases by the same customer. So, everything is working fine and perfectly normal.

Note that this error is expected to show when you're using an upsell funnel app like OCU or Carthook, as in this case any upsell and downsell conversions will trigger additional Purchase events.

Recommended action: Click the "Mark as Resolved" button at the bottom of the message.