sanger/sequencescape — pull request #514 by JamesGlover (2012-05-17 14:54:13+00:00)
Title: 29767381 order causes duplication of asset group asset associations

Avoids adding additional copies of assets to an asset
group on order validation. In addition, assumes that order
will only be dealing with receptacles

Includes adjusted test to use sample_tube rather than an asset