Communists have historically been the closest allies to anarchists. The fact that the two umbrellas have fought as well doesn't erase that there has been no other group anarchists or communists can claim as as close as each other.

When communists and anarchists do fight, it's usually in the context of civil war, or in the context of anarchist groups within socialist states working to undermine socialism and push for anarchism. The communists pushing back against this isn't "repressing anarchisn," so much as it is socialism protecting the existing system against those who would undermine it. The USSR supported the Spanish anarchists because at the time they shared common goals, but when anarchists organize for the overthrow of socialist states, that's when conflict arises.

We could flip this on its head just as easily: anarchism represses communism, in that both fundamentally come to opposite conclusions: anarchists seek full horizontalism and decentralization, while communists seek full collectivization, which means that though our goals are often fully aligned, they do diverge in the final analysis and as such no anarchist society would genuinely allow communism to be pushed for if it came to exist, and would have to oppress communists.