CoreBOSBB

I created a new custom field called "Valor líquido"  on Payments

II had not seen that it already existed a fields with the same name

System created a duplicate CF and not allowed a removed

How I solve that?

=> delete your custom field and create it with a new label.
=> change the label of the system field
=> hide the system field

Ok, only for registration at post

1 - not possible, system not shows a delete button ( only a cancel button )
2 - I think is directly in DB is possible .. ( I will try )
3 - Ok, works

You means change a field name in DB, in table vtiger_fields ?