Shady DOM distribution can fail for a dom-repeat inside a container with <content select="*">

[sorvell]

This is due to two issues: (1) dom-repeat re-inserts items when it is attached (it should only do so when it was previously detached, (2) Shady DOM logical children tracking does not correctly handle document fragments and can add children multiple times to a container's logical dom (fragment children should only be added if their logical location needs to change).