media:group must have multiple
From the specification [emphasis added]:
<media:group> is a sub-element of <item>. It allows grouping of <media:content> elements that are effectively the same content, yet different representations. For instance: the same song recorded in both the WAV and MP3 format. It's an optional element that must only be used for this purpose.
Omit the enclosing
<media:group> element in cases where
there is only one representation.
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