Using Sugar Open Source 4.5.1

It seems that when custom field data is entered a duplicate entry is posted to the database (ie: Leads_cstm > id_c)
I can't work out why this occurs and the duplicates can't be merged (error: "Please select at least 2 records to proceed").

I read a post suggesting that it's something to do with having multiple id fields in the DB - that is, 'id' in 'leads' table and 'id_c' in 'leads_cstm'.

I once tried to delete this and destroyed my database!!

I can manually remove the duplicates through the DB by searching th 'id_c' field and delting one by one - this just is not effective for 3,000 leads with most being duplicated 4-8 times each.

How do i stop leads (or contacts) from duplicating?

Desperate for some advice

Cheers.