diff --git a/server/test/shelf_index_postgres_test.gleam b/server/test/shelf_index_postgres_test.gleam
index 402676b..11a7f99 100644
--- a/server/test/shelf_index_postgres_test.gleam
+++ b/server/test/shelf_index_postgres_test.gleam
@@ -9,7 +9,9 @@ import crate_server/shelf_index_postgres
 import envoy
 import exception
 import gleam/erlang/process
+import gleam/int
 import gleam/option.{None, Some}
+import gleam/time/timestamp
 
 // A single-connection pool, killed as soon as this call returns (even if
 // `run` itself crashes): each test opens its own fresh pool, and a normal
@@ -59,6 +61,12 @@ fn blank_entry(
   )
 }
 
+fn fresh_did(tag: String) -> String {
+  let #(_, nanoseconds) =
+    timestamp.to_unix_seconds_and_nanoseconds(timestamp.system_time())
+  "did:plc:pgtest-" <> tag <> "-" <> int.to_string(nanoseconds)
+}
+
 pub fn event_round_trip_test() {
   use store <- with_store()
   let entry_uri = "at://did:plc:pgtest/dev.mokkenstorm.crate.shelf.entry/pg1"
@@ -120,7 +128,7 @@ pub fn record_and_fold_round_trips_flattened_columns_test() {
 
 pub fn seen_round_trip_test() {
   use store <- with_store()
-  let did = "did:plc:pgtest-seen"
+  let did = fresh_did("seen")
   assert store.seen.has_seen(did) == False
   store.seen.mark_seen(did)
   assert store.seen.has_seen(did) == True
@@ -128,7 +136,7 @@ pub fn seen_round_trip_test() {
 
 pub fn counts_reflect_stored_rows_and_go_back_down_on_delete_test() {
   use store <- with_store()
-  let did = "did:plc:pgtest-counts"
+  let did = fresh_did("counts")
   let entries_before = store.entries.count()
   let seen_before = store.seen.count()
   let entry_uri = "at://" <> did <> "/dev.mokkenstorm.crate.shelf.entry/pgc1"
@@ -157,7 +165,7 @@ pub fn counts_reflect_stored_rows_and_go_back_down_on_delete_test() {
 
 pub fn delete_for_did_round_trip_test() {
   use store <- with_store()
-  let did = "did:plc:pgtest-purge"
+  let did = fresh_did("purge")
   let entry_uri = "at://" <> did <> "/dev.mokkenstorm.crate.shelf.entry/pg3"
   shelf_index.record_and_fold(
     store,