erc-reuse-buffers

erc-reuse-buffers is a variable defined in `erc.el'.
Its value is

t

Documentation:
If nil, create new buffers on joining a channel/query.
If non-nil, a new buffer will only be created when you join
channels with same names on different servers, or have query buffers
open with nicks of the same name on different servers. Otherwise,
the existing buffers will be reused.

You can customize this variable.