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Dolly Parton 's not going to charge into the Colosseum at Caesar's Palace in December ... revealing her doctors tell her she must undergo multiple procedures.

The singer-songwriter revealed the news in a sweet Instagram post Sunday ... telling her fans she's still struggling with health problems -- and, she says "doctors tell me I must have a few procedures."

She cracks a joke at her own expense ... saying she told the docs it must be time for her 100K mile checkup -- lightening the mood a little with her classic wit.

However, Dolly says these procedures -- which she makes clear are not plastic surgery -- will prevent her from preparing for the shows properly ... and, so they've moved the dates. Gallery Launch Gallery Getty

She says people paid top dollar -- in fact, ti

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