Redundancy §36 in The Cooper Hill Stylebook

Example: If Ed Sullivan would have had black performers on his show in the 1950s, his ratings would have risen among minority viewers.

Problem: If and would have are both conditional. They both mean in case or on the condition that. Never use would in an if clause.

Correction: If Ed Sullivan had had black performers on his show in the 1950s, his ratings would have risen among minority viewers.

Note: had had is neither a typo nor a stutter — it is the correct way to indicate the past perfect of the verb to have.