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

There are only two good reasons - that I can think of - for allowing defining the same index twice

1. for compatibility with existing scripts that do define the same index twice.
2. changing the implementation would require work that I am neither willing to do nor pay for