diff --git a/modules/prebidServerBidAdapter/bidderConfig.js b/modules/prebidServerBidAdapter/bidderConfig.js
index 3b51810ff20..f6f4fb91389 100644
--- a/modules/prebidServerBidAdapter/bidderConfig.js
+++ b/modules/prebidServerBidAdapter/bidderConfig.js
@@ -52,7 +52,7 @@ export function extractEids({global, bidder}) {
   function getEntry(eid) {
     let entry = entries.find((candidate) => deepEqual(candidate.eid, eid));
     if (entry == null) {
-      entry = {eid, bidders: []}
+      entry = {eid, bidders: new Set()}
       entries.push(entry);
     }
     if (bySource[eid.source] == null) {
@@ -74,12 +74,12 @@ export function extractEids({global, bidder}) {
       (deepAccess(bidderConfig, path) || []).forEach(eid => {
         const entry = getEntry(eid);
         if (entry.bidders !== false) {
-          entry.bidders.push(bidderCode);
+          entry.bidders.add(bidderCode);
         }
       })
     })
   })
-  return {eids: entries, conflicts};
+  return {eids: entries.map(({eid, bidders}) => ({eid, bidders: bidders && Array.from(bidders)})), conflicts};
 }
 
 /**
diff --git a/test/spec/modules/prebidServerBidAdapter_spec.js b/test/spec/modules/prebidServerBidAdapter_spec.js
index 85ae7909f34..fc0f9ed7b68 100644
--- a/test/spec/modules/prebidServerBidAdapter_spec.js
+++ b/test/spec/modules/prebidServerBidAdapter_spec.js
@@ -2130,7 +2130,7 @@ describe('S2S Adapter', function () {
           bidders: [],
           source: 'idD'
         }]);
-      })
+      });
 
       it('should repeat global EIDs when bidder-specific EIDs conflict', () => {
         BID_REQUESTS.push({
@@ -4638,6 +4638,21 @@ describe('S2S Adapter', function () {
             ],
             conflicts: ['idA', 'idB']
           }
+        },
+        {
+          t: 'duplicated bidder-specific eids',
+          bidder: {
+            bidderA: {
+              user: {
+                eids: [mkEid('id'), mkEid('id')]
+              }
+            }
+          },
+          expected: {
+            eids: [
+              eidEntry('id', 'id', ['bidderA'])
+            ]
+          }
         }
       ].forEach(({t, global = {}, bidder = {}, expected}) => {
         it(t, () => {