Bugfix focuser duplicated abs position

Some focusers (like my FocusLynx) send regularly the current absolute position and not only if this position has changed. This creates problems during autofocusing, since finishing a move to a new position is detected by receiving an absolute position with the status IPS_OK.

This change makes the focuser more robust detecting multiple identical focuser positions.

Test plan

Testing this fix requires a focuser that shows this behavior. Without this change, there should occur multiple warning entries in the log file stating "Capture called while already in progress. Capture is ignored." With this fix, these warnings should no longer occur, nevertheless autofocusing should work.