diff --git a/Makefile b/Makefile
index 7c2c1b553e..ac61a61b0e 100644
--- a/Makefile
+++ b/Makefile
@@ -1,4 +1,8 @@
+# -DCOLLISION_CHECK if you believe that SHA1's
+# 1461501637330902918203684832716283019655932542976 hashes do not give you
+# enough guarantees about no collisions between objects ever hapenning.
 CFLAGS=-g -O3 -Wall
+
 CC=gcc
 
 PROG=   update-cache show-diff init-db write-tree read-tree commit-tree \
diff --git a/read-cache.c b/read-cache.c
index 5453694464..2ee96bc92c 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -216,8 +216,25 @@ int write_sha1_buffer(const unsigned char *sha1, void *buf, unsigned int size)
 	int fd;
 
 	fd = open(filename, O_WRONLY | O_CREAT | O_EXCL, 0666);
-	if (fd < 0)
-		return (errno == EEXIST) ? 0 : -1;
+	if (fd < 0) {
+		void *map;
+		static int error(const char * string);
+
+		if (errno != EEXIST)
+			return -1;
+#ifndef COLLISION_CHECK
+		fd = open(filename, O_RDONLY);
+		if (fd < 0)
+			return -1;
+		map = mmap(NULL, size, PROT_READ, MAP_PRIVATE, fd, 0);
+		if (map == MAP_FAILED)
+			return -1;
+		if (memcmp(buf, map, size))
+			return error("SHA1 collision detected!"
+					" This is bad, bad, BAD!\a\n");
+#endif
+		return 0;
+	}
 	write(fd, buf, size);
 	close(fd);
 	return 0;