I have seen this some times; 50 % of my customers have experienced this issue. It is easy to detect. Try to re-index all your databases (4). Re-index will fail if there are duplicate keys.

In most cases this statement fixed it.

DELETE FROM public.emailaddress AS A
USING public.emailaddress AS B
WHERE A.idemailaddress < B.idemailaddress
AND A.emailaddress = B.emailaddress;

However you have to change some settings if it is a different table.

Therefore here is a second example if table character_set is affected

DELETE FROM public.character_set AS A
USING public.character_set AS B
WHERE A.character_set_id < B.character_set_id
AND A.characterset = B.characterset;