Add useful error when duplicate web3 is detected.

Fixes #2507

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 ## Current Master
 
+- Detect when multiple web3 extensions are active, and provide useful error.
+
 ## 3.12.0 2017-10-25
 
 - Add support for alternative ENS TLDs (Ethereum Name Service Top-Level Domains).

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')