add changelog entry and docs for duplicate merging

docs/changelog.rst

@@ -6,6 +6,9 @@ Changelog
 
 New features:
 
+* The :doc:`/plugins/duplicates` plugin now allows merging duplicate
+  track metadata (when detecting duplicate items), or duplicate album
+  tracks (when detecting duplicate albums).
 * The :doc:`/plugins/duplicates` plugin now enforces an ordering on
   duplicates: it defaults to metadata attribute set completeness,
   or alternatively any list of attributes that should be favored.

docs/plugins/duplicates.rst

@@ -29,6 +29,7 @@ duplicates themselves via command-line switches ::
   -F, --full            show all versions of duplicate tracks or albums
   -s, --strict          report duplicates only if all attributes are set
   -k, --keys            report duplicates based on keys
+  -M, --merge           merge duplicate items
   -m DEST, --move=DEST  move items to dest
   -o DEST, --copy=DEST  copy items to dest
   -p, --path            print paths for matched items or albums
@@ -75,6 +76,8 @@ file. The available options mirror the command-line options:
   file, or as space-delimited strings in the command-line), you can extend this
   behavior to consider other attributes.
   Default: ``[mb_trackid, mb_albumid]``
+- **merge**: Merge duplicate items by consolidating tracks and-or
+  metadata where possible.
 - **move**: A destination base directory into which it will move matched
   items.
   Default: none (disabled).
@@ -145,4 +148,8 @@ Ignore items with undefined keys::
 
   beet duplicates --strict
 
+Merge and delete duplicate albums with different missing tracks::
+
+  beet duplicates --album --merge --delete
+
 .. _spark: https://github.com/holman/spark