You can cast a spell using flashback even if it was somehow put into your graveyard without having been cast.

The token created by Electroduplicate copies exactly what was printed on the original creature and nothing else, with the listed exceptions (unless that creature is copying something else or is a token; see below). It doesn't copy whether that creature is tapped or untapped, whether it has any counters on it or Auras and Equipment attached to it, or any non-copy effects that have changed its power, toughness, types, color, or so on.

If the copied creature is a token, the token that's created copies the original characteristics of that token as stated by the effect that created the token, with the listed exceptions.

If the copied creature is copying something else, then the token enters as whatever that creature copied, with the listed exceptions.

Any enters abilities of the copied creature will trigger when the token enters. Any "as [this permanent] enters" or "[this permanent] enters with" abilities of the copied creature will also work.

"Flashback [cost]" means "You may cast this card from your graveyard by paying [cost] rather than paying its mana cost" and "If the flashback cost was paid, exile this card instead of putting it anywhere else any time it would leave the stack."

You must still follow any timing restrictions and permissions, including those based on the card's type. For instance, you can cast a sorcery using flashback only when you could normally cast a sorcery.

To determine the total cost of a spell, start with the mana cost or alternative cost (such as a flashback cost) you're paying, add any cost increases, then apply any cost reductions. The mana value of the spell is determined only by its mana cost, no matter what the total cost to cast the spell was.

A spell cast using flashback will always be exiled afterward, whether it resolves, is countered, or leaves the stack in some other way.

If a card with flashback is put into your graveyard during your turn, you can cast it if it's legal to do so before any other player can take any actions.