There is no .azw3[1] format AFAIK. It is likely an anomaly introduced when a browser (or other software) sees a file of the same name already existing in the specified download directory.
The solution depends on what's causing the problem:
Confused browser? Change/upgrade browser.
Downloading more than once? Don't do that, or move/delete the original before re-downloading.
Every file sent down from the source is named the same? This can happen sometimes if the download link actually points to a program which spews forth the content (rather than provding a link to a discrete, downloadable file. Right-clicking the link would yield some generic name rather than a specific one.
Other: (waiting for onlookers to chime in)
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