diff --git a/crates/lox-library/src/dup_filter.rs b/crates/lox-library/src/dup_filter.rs
new file mode 100644
index 0000000..48681ed
--- /dev/null
+++ b/crates/lox-library/src/dup_filter.rs
@@ -0,0 +1,34 @@
+/*! Filter duplicate shows of credentials and open invitations by id
+ * (which will typically be a Scalar).  This implementation just keeps
+ * the table of seen ids in memory, but a production one would of course
+ * use a disk-backed database. */
+
+use std::collections::HashMap;
+use std::hash::Hash;
+use std::cmp::Eq;
+
+/// Each instance of DupFilter maintains its own independent table of
+/// seen ids.  IdType will typically be Scalar.
+#[derive(Default,Debug)]
+pub struct DupFilter<IdType> {
+    seen_table: HashMap<IdType, ()>,
+}
+
+impl<IdType: Hash + Eq + Copy> DupFilter<IdType> {
+    /// Check to see if the id is in the seen table, but do not add it
+    /// to the seen table.  Return true if it is already in the table,
+    /// false if not.
+    pub fn check(&self, id: &IdType) -> bool {
+        self.seen_table.contains_key(id)
+    }
+
+    /// As atomically as possible, check to see if the id is in the seen
+    /// table, and add it if not.  Return Ok(()) if it was not already
+    /// in the table, and Err(()) if it was.
+    pub fn filter(&mut self, id: &IdType) -> Result<(),()> {
+        match self.seen_table.insert(*id, ()) {
+            None => Ok(()),
+            Some(()) => Err(()),
+        }
+    }
+}