DES encryption - what happens if all sub-keys are the same? What will happen if we change the DES encryrption algorithm to use the same key in each round ?

How will it affect the encryption - will it only make the encryption same as decryption or are the more effects ? I couldn't find any other possible effects in the DES standard description.

Thanks
It will make the algorithm insecure.

It will be become vulnerable to slide attacks.

It will make encryption become equivalent to decryption, which is a weakness.  This is sometimes called a "weak key"; with your scheme, every key will be "weak".

It will also reduce the keyspace to 48 bits, which is too small to be secure.