...
Make sure that your starting number, is higher than your first ending number... such that...
Say you started at nonce 0, and plotted 2 million nonces... The next plot, make your starting nonce 2 million and 1, then you can plot another 2 million. Next one starting nonce 4 million and 5, and so forth.
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xxx_0_2000000_yyy (from 0 to 1999999)
xxx_2000000_2000000_yyy (from 2000000 to 3999999)
xxx_4000000_2000000_yyy (from 4000000 to 5999999)