[horde] duplicate key value violates unique constraint "horde_activesync_state_pkey"

[Sebastian Arcus]

I'm on Horde 5.2.17 with Lighttpd 1.4.49, PHP 5.6.35 and Postgresql 9.2.24

I get lots of the above errors in the logs and most of the 8 iPhones 
can't sync or download emails for several hours at a time, then they 
work again, they they stop again - and so on. The full error message is:

2018-05-14T09:11:11+01:00 ERR: HORDE SQL QUERY FAILED: SQLSTATE[23505]: 
Unique violation: 7 ERROR:  duplicate key value violates unique 
constraint "horde_activesync_state_pkey"
DETAIL:  Key (sync_key)=({5acb7873-baa4-4ea3-8f69-3d9bc0a83c02}1527) 
already exists.
	INSERT INTO "horde_activesync_state" ("sync_key", "sync_data",
	  "sync_devid", "sync_mod", "sync_folderid", "sync_user",
	  "sync_pending", "sync_timestamp") VALUES (?, :binary0, ?, ?, ?, ?, ?,
	  ?) [pid 11736 on line 282 of 
"/usr/lib/php/Horde/Db/Adapter/Pdo/Base.php"]

I would really appreciate some guidance on this please - as the mail 
downloading on the iPhones is really erratic and it drives users bonkers 
- and in turn they are driving me up the wall :)