Bifur, Melodic Rider

If Bifur entering causes you to have an enduring story, his "enters or attacks" ability triggers an additional time.

If a triggered ability is linked to a second ability, additional instances of that triggered ability are also linked to that second ability. If the second ability refers to "the exiled card," it refers to all cards exiled by instances of the triggered ability.

Once you have an enduring story, you have it for the rest of the game, even if you lose control of some or all of your storied permanents. The enduring story designation is on the player and can't be removed by any effect.

A permanent is any object on the battlefield, including tokens and lands. Spells and emblems aren't permanents.

Bifur's last ability doesn't copy the triggered ability; it just causes the ability to trigger an additional time. Any choices made as you put the ability onto the stack, such as modes and targets, are made separately for each instance of the ability. Any choices made on resolution, such as whether to put counters on a permanent, are also made individually.

If you control three legendary, Saga, and/or artifact permanents but don't control a permanent with storied, you don't get an enduring story. For example, if you controlled three artifacts, lost control of two, then cast Thorin Oakenshield, you won't have an enduring story.