facebook/relay · error

Can only insert '{}' once. Had {:?} and trying to insert {:?

Error message

Can only insert '{}' once. Had {:?} and trying to insert {:?}.

What it means

Panic in Program::insert_fragment: a fragment was inserted under a name that already exists in the program's fragment map. The previous and new fragment locations are printed. This invariant check fires during compiler transforms that add fragments (e.g. refetchable/connection transforms) when they generate or copy a fragment whose name collides with an existing one.

Source

Thrown at compiler/crates/graphql-ir/src/program.rs:80

                        panic!(
                            "\nDuplicate fragment definitions named {}: \nfirst one: {:?}\nsecond one: {:?}\n",
                            name, loc, &another.name.location
                        );
                    }
                }
            }
        }
        Self {
            schema,
            fragments,
            operations,
        }
    }

    pub fn insert_fragment(&mut self, fragment: Arc<FragmentDefinition>) {
        let name = fragment.name.item;
        if let Some(previous) = self.fragments.insert(name, fragment) {
            panic!(
                "Can only insert '{}' once. Had {:?} and trying to insert {:?}.",
                name, previous, self.fragments[&name]
            );
        };
    }

    pub fn fragment(&self, name: FragmentDefinitionName) -> Option<&Arc<FragmentDefinition>> {
        self.fragments.get(&name)
    }

    pub fn fragment_mut(
        &mut self,
        name: FragmentDefinitionName,
    ) -> Option<&mut Arc<FragmentDefinition>> {
        self.fragments.get_mut(&name)
    }

    /// Searches for an operation by name.

View on GitHub (pinned to 668b1b85e0)

Solutions

1. Check custom transforms that call insert_fragment for name collisions with existing fragments
2. Avoid defining a fragment whose name matches a compiler-generated one (e.g. from @refetchable or @connection)
3. If it reproduces with plain Relay code, report a compiler issue with the fragment definitions involved

When it happens

Trigger: Thrown at compiler/crates/graphql-ir/src/program.rs:80 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of facebook/relay@668b1b85e0 (2026-09-02). Data as JSON: /api/errors/dc5be1e95645ba79. Report an issue: GitHub.