I've had this problem before, several years ago and I had to create a new
table and reload my data to get rid of the problem then.
What is happening is very weird. I have 152 records in a table that have
somehow got another empty (phantom) record, in the same table, associated with them.

To find them I do a search for duplicate records using the "!" character
and I get 304 records returned but 152 are empty records. I can't add any
text into any fields in these records and can't delete them without crashing
Filemaker. However if I delete the associated records, the ones which do
have text in the mandatory fields these records dissappear.
However when I reload the 152 records these phantom records reappear.

If I try a global change on these 304 records the change only effects the
legit 152 and a message says the other 152 are either in use or I don't
have permission to alter them, neither is true of course.

Has anyone else experienced this problem? I don't want to have to reload a
new table, it'll take way too much time. Why would thes 152 records create
these ghost records? Last time it was only two records that did this and it
took ages to get rid of the problem by reloading the entire database. That
was years ago now and the database is exponentially bigger...

Help!

Hmm, phantom records that can't be deleted is a sure sign of corruption. You should recover what data you can from the file and revert to a recent backup that doesn't have the problem.

There's usually some circumstance that starts corruption, like sharing a file through the OS, or if a file is not closed properly. You should ensure that you're doing what you can to reduce these circumstances and also that you mitigate the damage by having regular backups.