Ignore duplicate interfaces from imports when using webpack loader

[mkochendorfer]

trafficstars

Fixes https://github.com/apollographql/graphql-tag/issues/194

[apollo-cla]

@mkochendorfer: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Meteor Contributor Agreement here: https://contribute.meteor.com/

[mkochendorfer]

Signed the agreement

[nblackburn]

Looks good, bumping this so it get's noticed.