Kate Middleton Look-Alike Heidi Agan Says Not Her in Video, Defends Princess

One of the UK's most popular Kate Middleton look-alikes is addressing the conspiracy theorists who think she's actually the woman in the new vid of Prince William and his wife -- and she has a solid alibi for her whereabouts.

Heidi Agan tells TMZ she was working at the head office of a dance school in Northamptonshire on Saturday -- which is about 80 miles from the Windsor Farm Shop. So, as Shaggy would put it ... Heidi says "it wasn't me" in that footage first posted Monday by TMZ.

Although it's not necessarily surprising conspiracy theorists are wondering if she was involved ... as Heidi says she was confused for being Kate during a recent outing -- she says the place erupted when people thought the MIA princess had surfaced.

And, Heidi has been busier than ever thanks to the constant speculation regarding Kate's whereabouts. The pro look-alike says normally her work is busy when the royals are busy -- whether that's because of weddings, pregnancies, etc.

However, despite Kate stepping back from the spotlight, Heidi claims she still has a lot of requests coming in.

As for how she feels about the conspiracy theories -- Heidi thinks the speculation has gone a bit too far and has sympathy for the Duchess of Wales. She personally thinks Kate is fine and is simply recovering from abdominal surgery ... just as the Palace announced back in January.

While Kate is a public figure, Heidi believes the royal has a right to her privacy when it comes to her medical records, and reminds fans the Princess isn't public property.

She points to the lack of information as the fuel for the increasingly wild theories ... especially since King Charles was so forthcoming with his cancer diagnosis.

The Mother's Day photo was just icing on the cake ... since it was likely meant to squash any concern, but did the exact opposite.