bogus rel-canonical results in inappropriate coalescence

### Steps to reproduce

Annotate (in a fresh group) the three URLs mentioned in https://hypothes-is.slack.com/archives/C03QZM0K4/p1491481119704670, namely

http://digitallearning.middcreate.net/summit/reflections-on-the-digital/five-ways-of-looking-at-the-digital-reflections-on-false-binaries/

http://digitallearning.middcreate.net/middcreate/the-digital-is-about-doing/

http://digitallearning.middcreate.net/critical-digital-pedagogy/exploring-digital-sanctuary/

### Expected behaviour
An AP card for each of the 3 URLs

### Actual behaviour
Two URLs share a card:


### Additional details

Slack: https://hypothes-is.slack.com/archives/C03QZM0K4/p1491481119704670

Drive: https://docs.google.com/document/d/1R094mP9j5sQSLDQasoFBl1JqWk6Jkdh_DxJLDhGSkT8/edit