diff --git a/bip/src/bp.rs b/bip/src/bp.rs
index 09d219e..14005cf 100644
--- a/bip/src/bp.rs
+++ b/bip/src/bp.rs
@@ -28,6 +28,7 @@ pub enum Error {
     BadWrite(std::io::Error),
     NotDefined,
     ExpectedID,
+    RedefData,
 }
 
 impl fmt::Display for Error {
@@ -37,6 +38,7 @@ impl fmt::Display for Error {
             Self::BadWrite(e) => write!(f, "{}", e),
             Self::NotDefined => write!(f, "undefined data type"),
             Self::ExpectedID => write!(f, "expected identifier"),
+            Self::RedefData => write!(f, "redefining existing datatype"),
         }
     }
 }
@@ -189,7 +191,9 @@ impl Writer {
                     if s == "data" {
                         let lhs = lhs.clone();
                         let data = self.parse_data_type(rhs);
-                        self.data.insert(lhs, data);
+                        self.data
+                            .insert(lhs, data)
+                            .map_or(Ok(()), |_| Err(Error::RedefData))?;
                     } else {
                         self.parse_id(rhs)?;
                     }