diff --git a/mercurial/copies.py b/mercurial/copies.py
index 36bf44d476666f834d3a1795dfef579a19025f42_bWVyY3VyaWFsL2NvcGllcy5weQ==..31c50763ff6f1e3fcaeba506e48b0dbf33cdc766_bWVyY3VyaWFsL2NvcGllcy5weQ== 100644
--- a/mercurial/copies.py
+++ b/mercurial/copies.py
@@ -405,5 +405,7 @@
                 # changeset based copies. It was made without regards with
                 # potential filelog related behavior.
                 if parent == 1:
+                    if newcopies is copies:
+                        newcopies = copies.copy()
                     minor, major = othercopies, newcopies
                 else:
@@ -408,6 +410,9 @@
                     minor, major = othercopies, newcopies
                 else:
-                    minor, major = newcopies, othercopies
+                    # we do not know if the other dict is a copy or not, so we
+                    # need to blindly copy it. Future change should make this
+                    # unnecessary.
+                    minor, major = newcopies, othercopies.copy()
                 copies = _merge_copies_dict(minor, major, isancestor, changes)
                 all_copies[c] = copies