diff --git a/tests/warnings/Mmakefile b/tests/warnings/Mmakefile
index 480e2e206..0e069a481 100644
--- a/tests/warnings/Mmakefile
+++ b/tests/warnings/Mmakefile
@@ -92,6 +92,10 @@ ERRORCHECK_PROGS = \
 #
 #	profiling_builtin
 #	time
+#
+# In fact, the mere presence of these files interferes with all the other
+# tests in this directory in grades that use mmc --make, so all their files
+# now have an added lib_dup_ prefix.
 
 ifeq ($(MMAKE_USE_MMC_MAKE),yes)
 	ERRORCHECK_PROGS += unused_args_analysis
diff --git a/tests/warnings/profiling_builtin.err_exp b/tests/warnings/lib_dup_profiling_builtin.err_exp
similarity index 100%
rename from tests/warnings/profiling_builtin.err_exp
rename to tests/warnings/lib_dup_profiling_builtin.err_exp
diff --git a/tests/warnings/profiling_builtin.m b/tests/warnings/lib_dup_profiling_builtin.m
similarity index 100%
rename from tests/warnings/profiling_builtin.m
rename to tests/warnings/lib_dup_profiling_builtin.m
diff --git a/tests/warnings/time.err_exp b/tests/warnings/lib_dup_time.err_exp
similarity index 100%
rename from tests/warnings/time.err_exp
rename to tests/warnings/lib_dup_time.err_exp
diff --git a/tests/warnings/time.m b/tests/warnings/lib_dup_time.m
similarity index 100%
rename from tests/warnings/time.m
rename to tests/warnings/lib_dup_time.m
diff --git a/tests/warnings/time.subtime.m b/tests/warnings/lib_dup_time.subtime.m
similarity index 100%
rename from tests/warnings/time.subtime.m
rename to tests/warnings/lib_dup_time.subtime.m