Allow multiple import of a module

Multiple import are treated as one. Produce a warning message.

tests/test_ASTModulesImporter.cpp

@@ -59,6 +59,20 @@ import math;
     CHECK_AST(data, result);
   }
 
+  SECTION("module multiple import")
+  {
+    std::string_view data = R"(
+import math;
+import math;
+)";
+
+    std::string_view result = R"(
+(root:undefined)
+)";
+
+    CHECK_AST(data, result);
+  }
+
   SECTION("error")
   {
     SECTION("unknown module")