Why most SQL databases allow defining the same index twice?

Question

Why most SQL databases allow defining the same index (or constraint) twice?

For example in MySQL I can do:

CREATE TABLE testkey(id VARCHAR(10) NOT NULL,          


        

Answer 1

You shouldn't be in a scenario that you have so many indexes on a table that you can't just quickly look and see if the index in there.

As for naming constraints and indexes, I only really ever name constraints. I will name a constraint FK_CurrentTable_ForeignKeyedColumn, just so things are more visible when quickly looking through lists of them.