The security and integrity of the Duolingo English Test is our highest priority. On very rare occasions, we detect additional information after a test was certified and shared with an institution that triggers another round of security checks, and a reversal of certification. This only happens when we detect intentionally malicious or misrepresentative behavior on the tester's behalf.
We understand that a decision to reverse certification adds complexity to institutional processes, but we believe we are obligated to inform any institution involved for the sake of transparency and integrity.
When a test is decertified, we email the test taker about the decertification and the reasons behind it. We remove their access to their test certificate, block them from taking the test again if appropriate, and tell them any institution they shared the result with has been informed. Here is the FAQ article for test takers regarding decertification.