Fix activated_path_skip on repeated running of emsdk_env.sh

The first time around `node` was being correctly added to the PATH, but
the second time around this code was observing the emsdk copy of node
in the PATH and assuming it could be skipped.

Fixes: #1240

emsdk.py

@@ -1438,9 +1438,18 @@ def get_required_path(active_tools):
   path_add = [to_native_path(EMSDK_PATH)]
   for tool in active_tools:
     if hasattr(tool, 'activated_path'):
-      if hasattr(tool, 'activated_path_skip') and which(tool.activated_path_skip):
-        continue
       path = to_native_path(tool.expand_vars(tool.activated_path))
+      # If the tool has an activated_path_skip attribute then we don't add
+      # the tools path to the users path if a program by that name is found
+      # in the existing PATH.  This allows us to, for example, add our version
+      # node to the users PATH if, and only if, they don't already have a
+      # another version of node in thier PATH.
+      if hasattr(tool, 'activated_path_skip'):
+        current_path = which(tool.activated_path_skip)
+        # We found an executable by this name in the current PATH, but we
+        # ignore our own version for this purpose.
+        if current_path and os.path.dirname(current_path) != path:
+          continue
       path_add.append(path)
   return path_add