erc-timestamp-only-if-changed-flag

erc-timestamp-only-if-changed-flag is a variable defined in `erc-stamp.el'.
Its value is

t

Documentation:
Insert timestamp only if its value changed since last insertion.
If `erc-insert-timestamp-function' is `erc-insert-timestamp-left', a
string of spaces which is the same size as the timestamp is added to
the beginning of the line in its place. If you use
`erc-insert-timestamp-right', nothing gets inserted in place of the
timestamp.

You can customize this variable.