document duplicate filter's ALL option

diff --git a/gpsbabel/README b/gpsbabel/README
index 9d1e9d65dad7c9f1b6762188c8cd754b24650e7f..86b625116b5fe766d30daed875ee27ffe9af286f 100644 (file)
--- a/gpsbabel/README
+++ b/gpsbabel/README
@@ -467,8 +467,9 @@ DATA FILTERS
        The duplicate filter is designed to remove duplicate points based
        on their shortname (traditionally a waypoint's name on the GPS
        receiver), and/or their location (to a precision of 6 decimals).
-       This filter supports two options, "shortname" and "location".
-       Generally, at least one of these options is REQUIRED.  For example:
+       This filter supports two options that specify how duplicates will
+        be recognized, "shortname" and "location".  Generally, at least one 
+        of these options is REQUIRED.  For example:
 
        gpsbabel -i gpx -f 1.gpx -f 2.gpx -x duplicate,location,shortname \
                 -o gpx -F merged_with_no_dupes.gpx
@@ -477,6 +478,14 @@ DATA FILTERS
        locations.  The result would be a GPX file that more than likely
        contains only unique points and point data.
 
+        The duplicate filter can also take an "all" option.  If you specify 
+        that option, all instances of a duplicated waypoint will be removed,
+        not just the second and subsequent instances.  If your input file
+        contains waypoints A, B, B, and C, the output file will contain 
+        waypoints A, B, and C without the "all" option, or just A and C 
+        with the "all" option.  This option can be useful as an "ignore
+        list" in some circumstances.
+
     DISTANCE ALONG A ROUTE (ARC)
     ARC