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do_feed_* are not filter hooks, they are action hooks.

Their purpose is not to modify generated markup, but only to call function that loads template. It is highly recommended to only have single function hooked to these.

So correct way to use this:

1. Remove native function that loads feed template.
2. Create your tweaked template.
3. Add your own function that loads your template.

Alternatively use filters in template tags and is_feed() conditional to modify their output when producing feed.