The problem is this . The bolt and insert spin when attempting to remove the coil cover so the "mechanic" pulls on the coil cover, brakes the lug and then he/she can get at the bolt and pull it out with the insert.
He cleans it up puts some epoxy on it and forces it back into the hole then inserts the bolt. The bolt acts like a piston and pushes the epoxy and insert up.
this means the plug cover is not seated on its gasket and not waterproof and we get water in the plug holes.
To Repair.
You need these bits.

If you can get the insert 1 out you can re use it .Pull it out with a long 5mm bolt 2 or if its well stuck put a small soldering Iron in the hole and melt the epoxy. 5-10 min.
Clean out the hole well with a drill / Dremmel/ thinners (not white spirit).
Grease the bolt threads and fit to the insert. epoxy it up and fit to hole flush with the top surface. Wait for it to set. Take out the long bolt and ensure no protrusions.

What I used is item 3 called a rivit nut. Intended to put threaded insert into thin sheet. This is only threaded at one end so I fit it with the flange at the bottom of the hole /threads at the top. Grease the bolt ,put it all the way in and glue into place .let it set and remove the bolt.
For bolts I have used item 4 Big heads /allen key ,stainless cut to length as required.