diff --git a/www/manager6/form/IPRefSelector.js b/www/manager6/form/IPRefSelector.js
index 08fafd9..7be09b6 100644
--- a/www/manager6/form/IPRefSelector.js
+++ b/www/manager6/form/IPRefSelector.js
@@ -8,6 +8,9 @@ Ext.define('PVE.form.IPRefSelector', {
 
     ref_type: undefined, // undefined = any [undefined, 'ipset' or 'alias']
 
+    // list of references that should not be offered, e.g. because they are already in use
+    excludeRefs: [],
+
     valueField: 'scopedref',
     displayField: 'ref',
     notFoundIsValid: true,
@@ -54,6 +57,7 @@ Ext.define('PVE.form.IPRefSelector', {
                 property: 'ref',
                 direction: 'ASC',
             },
+            filters: [(rec) => !me.excludeRefs.includes(rec.data.scopedref)],
         });
 
         var columns = [];
diff --git a/www/manager6/panel/IPSet.js b/www/manager6/panel/IPSet.js
index 9e0203a..d510ae6 100644
--- a/www/manager6/panel/IPSet.js
+++ b/www/manager6/panel/IPSet.js
@@ -187,6 +187,9 @@ Ext.define('PVE.IPSetCidrEdit', {
 
     cidr: undefined,
 
+    // entries already in the IPSet, not offered again when adding a new one
+    existingCidrs: [],
+
     initComponent: function () {
         var me = this;
 
@@ -211,6 +214,7 @@ Ext.define('PVE.IPSetCidrEdit', {
                 xtype: 'pveIPRefSelector',
                 name: 'cidr',
                 ref_type: 'alias',
+                excludeRefs: me.existingCidrs,
                 autoSelect: false,
                 editable: true,
                 base_url: me.list_refs_url,
@@ -359,6 +363,7 @@ Ext.define(
                     var win = Ext.create('PVE.IPSetCidrEdit', {
                         base_url: me.base_url,
                         list_refs_url: me.list_refs_url,
+                        existingCidrs: store.collect('cidr'),
                     });
                     win.show();
                     win.on('destroy', reload);