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Since your return is for all practical purposes filed, you will need to file an amended return to include the dividend income. It may result in an increased tax liability, or possibly not at all for the federal return--as some qualifying dividends are taxed at 0% if a person is in the 15% tax bracket.
The amended return is Form 1040X. http://www.irs.gov/pub/irs-pdf/f1040x.pdf
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