diff --git a/modules/gdscript/gdscript_parser.cpp b/modules/gdscript/gdscript_parser.cpp
index ddf880d36c7..74f07e7b1d3 100644
--- a/modules/gdscript/gdscript_parser.cpp
+++ b/modules/gdscript/gdscript_parser.cpp
@@ -3686,6 +3686,11 @@ void GDScriptParser::_parse_class(ClassNode *p_class) {
 						_add_warning(GDScriptWarning::FUNCTION_CONFLICTS_VARIABLE, -1, name);
 					}
 				}
+				for (int i = 0; i < p_class->subclasses.size(); i++) {
+					if (p_class->subclasses[i]->name == name) {
+						_add_warning(GDScriptWarning::FUNCTION_CONFLICTS_CONSTANT, -1, name);
+					}
+				}
 #endif // DEBUG_ENABLED
 
 				if (tokenizer->get_token() != GDScriptTokenizer::TK_PARENTHESIS_OPEN) {
@@ -4630,6 +4635,13 @@ void GDScriptParser::_parse_class(ClassNode *p_class) {
 						return;
 					}
 				}
+
+				for (int i = 0; i < current_class->subclasses.size(); i++) {
+					if (current_class->subclasses[i]->name == member.identifier) {
+						_set_error("A class named '" + String(member.identifier) + "' already exists in this class (at line " + itos(current_class->subclasses[i]->line) + ").");
+						return;
+					}
+				}
 #ifdef DEBUG_ENABLED
 				for (int i = 0; i < current_class->functions.size(); i++) {
 					if (current_class->functions[i]->name == member.identifier) {
@@ -4874,6 +4886,13 @@ void GDScriptParser::_parse_class(ClassNode *p_class) {
 					}
 				}
 
+				for (int i = 0; i < current_class->subclasses.size(); i++) {
+					if (current_class->subclasses[i]->name == const_id) {
+						_set_error("A class named '" + String(const_id) + "' already exists in this class (at line " + itos(current_class->subclasses[i]->line) + ").");
+						return;
+					}
+				}
+
 				tokenizer->advance();
 
 				if (tokenizer->get_token() == GDScriptTokenizer::TK_COLON) {
@@ -4944,6 +4963,13 @@ void GDScriptParser::_parse_class(ClassNode *p_class) {
 						}
 					}
 
+					for (int i = 0; i < current_class->subclasses.size(); i++) {
+						if (current_class->subclasses[i]->name == enum_name) {
+							_set_error("A class named '" + String(enum_name) + "' already exists in this class (at line " + itos(current_class->subclasses[i]->line) + ").");
+							return;
+						}
+					}
+
 					tokenizer->advance();
 				}
 				if (tokenizer->get_token() != GDScriptTokenizer::TK_CURLY_BRACKET_OPEN) {
@@ -5029,6 +5055,13 @@ void GDScriptParser::_parse_class(ClassNode *p_class) {
 								}
 							}
 
+							for (int i = 0; i < current_class->subclasses.size(); i++) {
+								if (current_class->subclasses[i]->name == const_id) {
+									_set_error("A class named '" + String(const_id) + "' already exists in this class (at line " + itos(current_class->subclasses[i]->line) + ").");
+									return;
+								}
+							}
+
 							ClassNode::Constant constant;
 							constant.type.has_type = true;
 							constant.type.kind = DataType::BUILTIN;