erc-reuse-frames

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

t

Documentation:
Determines whether new frames are always created.
Non-nil means that a new frame is not created to display an ERC
buffer if there is already a window displaying it. This only has
effect when `erc-join-buffer' is set to `frame'.

You can customize this variable.