I've created a simple low level discovery rule:
UserParameter=mysql.slave.discovery, echo '{"data":[{"

{#SLAVE}":"slave"}]}'
I've also added some item prototypes. They do not include {#SLAVE}

macros as I just need to run some checks if the host is in a slave role. I do not have any filer rules. Everything is set up for a template.

Now I've linked a host and get the following error:
Cannot create item: item with the same key "mysql.replicsec" already exists.
But it doesn't! If I add a macro for an item key it actually gets created, but the error doesn't go away.