diff --git a/src/etc/errorck.py b/src/etc/errorck.py
index 4b1b78da9fde4..48736542f20c7 100644
--- a/src/etc/errorck.py
+++ b/src/etc/errorck.py
@@ -23,6 +23,18 @@
 errcode_map = {}
 error_re = re.compile("(E\d\d\d\d)")
 
+# In the register_long_diagnostics! macro, entries look like this:
+#
+# EXXXX: r##"
+# <Long diagnostic message>
+# "##,
+#
+# These two variables are for detecting the beginning and end of diagnostic
+# messages so that duplicate error codes are not reported when a code occurs
+# inside a diagnostic message
+long_diag_begin = "r##\""
+long_diag_end = "\"##"
+
 for (dirpath, dirnames, filenames) in os.walk(src_dir):
     if "src/test" in dirpath or "src/llvm" in dirpath:
         # Short circuit for fast
@@ -35,7 +47,14 @@
         path = os.path.join(dirpath, filename)
 
         with open(path, 'r') as f:
+            inside_long_diag = False
             for line_num, line in enumerate(f, start=1):
+                if inside_long_diag:
+                    # Skip duplicate error code checking for this line
+                    if long_diag_end in line:
+                        inside_long_diag = False
+                    continue
+
                 match = error_re.search(line)
                 if match:
                     errcode = match.group(1)
@@ -47,6 +66,9 @@
                     else:
                         errcode_map[errcode] = new_record
 
+                if long_diag_begin in line:
+                    inside_long_diag = True
+
 errors = False
 all_errors = []