diff --git a/candran.can b/candran.can
index 899dd31..27d8c1b 100644
--- a/candran.can
+++ b/candran.can
@@ -38,10 +38,13 @@ function candran.preprocess(input, options={})
 		i += 1
 		if line:match("^%s*#") and not line:match("^#!") then -- exclude shebang
 			preprocessor ..= line:gsub("^%s*#", "")
-		elseif options.mapLines then
-			preprocessor ..= ("write(%q)"):format(line:sub(1, -2) .. " -- "..options.chunkname..":" .. i) .. "\n"
 		else
-			preprocessor ..= ("write(%q)"):format(line:sub(1, -2)) .. "\n"
+			local l = line:sub(1, -2)
+			if options.mapLines and not l:match("%-%- (.-)%:(%d+)$") then
+				preprocessor ..= ("write(%q)"):format(l .. " -- "..options.chunkname..":" .. i) .. "\n"
+			else
+				preprocessor ..= ("write(%q)"):format(line:sub(1, -2)) .. "\n"
+			end
 		end
 	end
 	preprocessor ..= "return output"
diff --git a/candran.lua b/candran.lua
index 3704f58..005d070 100644
--- a/candran.lua
+++ b/candran.lua
@@ -1922,14 +1922,17 @@ for line in (input .. "\
 i = i + 1
 if line:match("^%s*#") and not line:match("^#!") then
 preprocessor = preprocessor .. line:gsub("^%s*#", "")
-elseif options["mapLines"] then
-preprocessor = preprocessor .. ("write(%q)"):format(line:sub(1, - 2) .. " -- " .. options["chunkname"] .. ":" .. i) .. "\
+else
+local l = line:sub(1, - 2)
+if options["mapLines"] and not l:match("%-%- (.-)%:(%d+)$") then
+preprocessor = preprocessor .. ("write(%q)"):format(l .. " -- " .. options["chunkname"] .. ":" .. i) .. "\
 "
 else
 preprocessor = preprocessor .. ("write(%q)"):format(line:sub(1, - 2)) .. "\
 "
 end
 end
+end
 preprocessor = preprocessor .. "return output"
 local env = util["merge"](_G, options)
 env["candran"] = candran