Add useful error when duplicate web3 is detected.

Fixes #2507

1 files changed, 7 insertions, 0 deletions

diff --git a/app/scripts/inpage.js b/app/scripts/inpage.js
index b6889b00f..9261e7d64 100644
--- a/app/scripts/inpage.js
+++ b/app/scripts/inpage.js
@@ -31,6 +31,13 @@ var inpageProvider = new MetamaskInpageProvider(metamaskStream)
 // setup web3
 //
 
+if (typeof window.web3 !== 'undefined') {
+  throw new Error(`MetaMask detected another web3.
+     MetaMask will not work reliably with another web3 extension.
+     This usually happens if you have two MetaMasks installed,
+     or MetaMask and another web3 extension. Please remove one
+     and try again.`)
+}
 var web3 = new Web3(inpageProvider)
 web3.setProvider = function () {
   log.debug('MetaMask - overrode web3.setProvider')