bug: Close() gets called more than once

[awlx]

When a client hard resets the connection with a connection reset by peer the Close() function gets called twice which makes acting here a bit harder :).

[fionera]

Based on https://pkg.go.dev/io#Closer

The behavior of Close after the first call is undefined. Specific implementations may document their own behavior.

Yes I think this shouldn't happen, having a sync.Once there would help, but dunno if I would call this a bug 😄