diff --git a/rest.js b/rest.js
index 68be741..b4c9408 100644
--- a/rest.js
+++ b/rest.js
@@ -186,7 +186,11 @@ module.exports.create = function (bigconf, deps, app) {
       return req.Store.find({ azpSub: req.body.sub });
     }).then(function (existing) {
       if (existing.length) {
-        throw new Error("PPID collision detected, cannot save authorized party's sub");
+        if (existing.length > 1) {
+          throw new Error("pre-existing PPID collision detected");
+        } else if (existing[0].sub !== req.params.sub || existing[0].azp !== req.params.azp) {
+          throw new Error("PPID collision detected, cannot save authorized party's sub");
+        }
       }
 
       var grant = {