diff --git a/crates/nu-parser/src/errors.rs b/crates/nu-parser/src/errors.rs
index f55fed2da7..444873d1ae 100644
--- a/crates/nu-parser/src/errors.rs
+++ b/crates/nu-parser/src/errors.rs
@@ -77,6 +77,10 @@ pub enum ParseError {
     #[diagnostic(code(nu::parser::module_not_found), url(docsrs))]
     ModuleNotFound(#[label = "module not found"] Span),
 
+    #[error("Duplicate command definition within a block.")]
+    #[diagnostic(code(nu::parser::duplicate_command_def), url(docsrs))]
+    DuplicateCommandDef(#[label = "defined more than once"] Span),
+
     #[error("Unknown command.")]
     #[diagnostic(
         code(nu::parser::unknown_command),
diff --git a/crates/nu-parser/src/parse_keywords.rs b/crates/nu-parser/src/parse_keywords.rs
index 3195acd785..6eb40daad0 100644
--- a/crates/nu-parser/src/parse_keywords.rs
+++ b/crates/nu-parser/src/parse_keywords.rs
@@ -13,7 +13,7 @@ use crate::{
     ParseError,
 };
 
-pub fn parse_def_predecl(working_set: &mut StateWorkingSet, spans: &[Span]) {
+pub fn parse_def_predecl(working_set: &mut StateWorkingSet, spans: &[Span]) -> Option<ParseError> {
     let name = working_set.get_span_contents(spans[0]);
 
     // handle "export def" same as "def"
@@ -42,9 +42,13 @@ pub fn parse_def_predecl(working_set: &mut StateWorkingSet, spans: &[Span]) {
             signature.name = name;
             let decl = signature.predeclare();
 
-            working_set.add_predecl(decl);
+            if working_set.add_predecl(decl).is_some() {
+                return Some(ParseError::DuplicateCommandDef(spans[1]));
+            }
         }
     }
+
+    None
 }
 
 pub fn parse_def(
@@ -98,7 +102,7 @@ pub fn parse_def(
                         (&name, signature, block_id)
                     {
                         let decl_id = working_set
-                            .find_predecl(name.as_bytes())
+                            .find_decl(name.as_bytes())
                             .expect("internal error: predeclaration failed to add definition");
 
                         let declaration = working_set.get_decl_mut(decl_id);
diff --git a/crates/nu-parser/src/parser.rs b/crates/nu-parser/src/parser.rs
index d3464869f4..79213fdf7b 100644
--- a/crates/nu-parser/src/parser.rs
+++ b/crates/nu-parser/src/parser.rs
@@ -2606,16 +2606,18 @@ pub fn parse_block(
         working_set.enter_scope();
     }
 
+    let mut error = None;
+
     // Pre-declare any definition so that definitions
     // that share the same block can see each other
     for pipeline in &lite_block.block {
         if pipeline.commands.len() == 1 {
-            parse_def_predecl(working_set, &pipeline.commands[0].parts);
+            if let Some(err) = parse_def_predecl(working_set, &pipeline.commands[0].parts) {
+                error = error.or(Some(err));
+            }
         }
     }
 
-    let mut error = None;
-
     let block: Block = lite_block
         .block
         .iter()
diff --git a/crates/nu-protocol/src/engine/engine_state.rs b/crates/nu-protocol/src/engine/engine_state.rs
index e1dcb7024d..0653ed73c9 100644
--- a/crates/nu-protocol/src/engine/engine_state.rs
+++ b/crates/nu-protocol/src/engine/engine_state.rs
@@ -311,17 +311,13 @@ impl<'a> StateWorkingSet<'a> {
         decl_id
     }
 
-    pub fn add_predecl(&mut self, decl: Box<dyn Command>) {
+    pub fn add_predecl(&mut self, decl: Box<dyn Command>) -> Option<DeclId> {
         let name = decl.name().as_bytes().to_vec();
 
         self.delta.decls.push(decl);
         let decl_id = self.num_decls() - 1;
 
-        self.delta.predecls.insert(name, decl_id);
-    }
-
-    pub fn find_predecl(&mut self, name: &[u8]) -> Option<DeclId> {
-        self.delta.predecls.get(name).copied()
+        self.delta.predecls.insert(name, decl_id)
     }
 
     pub fn merge_predecl(&mut self, name: &[u8]) -> Option<DeclId> {