avoid duplicate alias sign up

services/gunDB/Mediator/index.js

@@ -1275,6 +1275,38 @@ const register = async (alias, pass) => {
     )
   }
 
+  // this import is done here to avoid circular dependency hell
+  const { timeout5 } = require('../contact-api/utils')
+
+  let userData = await timeout5(
+    new Promise(res => {
+      gun.get(`~@${alias}`).once(ud => res(ud))
+    })
+  )
+
+  if (userData) {
+    throw new Error(
+      'The given alias has been used before, use an unique alias instead.'
+    )
+  }
+
+  await new Promise(res => setTimeout(res, 300))
+
+  userData = await timeout5(
+    new Promise(res => {
+      gun.get(`~@${alias}`).once(ud => res(ud), {
+        // https://github.com/amark/gun/pull/971#issue-438630761
+        wait: 1500
+      })
+    })
+  )
+
+  if (userData) {
+    throw new Error(
+      'The given alias has been used before, use an unique alias instead. (Caught at 2nd try)'
+    )
+  }
+
   _isRegistering = true
 
   /** @type {import('../contact-api/SimpleGUN').CreateAck} */
@@ -1282,6 +1314,13 @@ const register = async (alias, pass) => {
     user.create(alias, pass, ack => res(ack))
   )
 
+  // An empty ack object seems to be caused by a duplicate alias sign up
+  if ('{}' === JSON.stringify(ack)) {
+    throw new Error(
+      'The given alias has been used before, use an unique alias instead. (Empty ack)'
+    )
+  }
+
   _isRegistering = false
 
   if (typeof ack.err === 'string') {