Junio C Hamano <gitster@pobox.com> writes:

quoted text
> Having said that, I think what is happening is that the final set of
> "other parents" is computed inside git-merge out of MERGE_HEAD and that is
> usually what is recorded in the resulting merge, but if the merge results
> in a conflict with manual resolution, that information is not given to the
> final "git commit".  The resulting commit records the parents out of HEAD
> and MERGE_HEAD.  I do not think this part has changed from scripted
> version of git-commit.

Sorry, my thinko.

The scripted version obviously used commit-tree to omit the duplicated
parent.  Perhaps we can do something like this.

-- >8 --
commit: drop duplicated parents

The scripted version of git-commit internally used git-commit-tree which
omitted duplicated parents given from the command line.  This prevented a
nonsensical octopus merge from getting created even when you said "git
merge A B" while you are already on branch A.

However, when git-commit was rewritten in C, this sanity check was lost.
This resurrects it.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 builtin-commit.c |    9 +++++++++
 1 files changed, 9 insertions(+), 0 deletions(-)

diff --git a/builtin-commit.c b/builtin-commit.c
index b294c1f..1d8d208 100644
--- a/builtin-commit.c
+++ b/builtin-commit.c
@@ -883,10 +883,19 @@ static void add_parent(struct strbuf *sb, const unsigned char *sha1)
 {
 	struct object *obj = parse_object(sha1);
 	const char *parent = sha1_to_hex(sha1);
+	const char *cp;
+
 	if (!obj)
 		die("Unable to find commit parent %s", parent);
 	if (obj->type != OBJ_COMMIT)
 		die("Parent %s isn't a proper commit", parent);
+	cp = strstr(sb->buf, parent);
+	if (cp &&
+	    sb->buf + 8 <= cp && !memcmp(cp - 8, "\nparent ", 8) &&
+	    cp[40] == '\n') {
+		error("duplicate parent %s ignored", parent);
+		return;
+	}
 	strbuf_addf(sb, "parent %s\n", parent);
 }
 
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