there isn't a double taxation. the 1099-B does not have the correct tax basis. that comes from the sales schedule the PTP would have provided as part of the K-1. go through the sales schedule carefully because any ordinary income that needs to be reported adds to your tax basis.
an example: numbers refer to column number from a typical sales schedule
1) unit sold
2) sale date (purchase date comes from 1099-B)
3) sales proceeds (from 1099-B)
4) purchase price
5) cumulative adjustments to tax basis
6) cost basis (should be the same as tax basis but if you inherited the PTP or acquired it other than through a direct purchase it tax basis could be something else) this should be column4 + column 5
7) gain subject to recapture as ordinary income. this is what gets entered on the PTP k-1 sales schedule as the sales price and then needs to be entered in the box for ordinary income. cost is 0 if you don't do the sales section of the k-1 any suspended PTP losses will not be allowed. you'll know you made the correct entries if this amount shows up on form 4797 line 10
your tax basis is column 6 + 7 this is what goes on form 1099-B - the capital gain sales schedule
for some PTP's it's possible that there is no column 7 or the amount is 0. you still need to enter the sales info in the k-1 sales page for this (just use 0 as sales price) so any suspended losses are allowed
there may be additional columns for:
the alternative minimum tax (AMT) adjustment which will produce a different capital gain for AMT purposes
% long term or short term (the capital gain/loss amount)
depreciation adjustments for nonconforming states.
you'll have to deal with these columns yourself because it depends on what you entered from the k-1's you received in past years for this PTP.
finally, there may be a state reporting page. can't say for sure but most likely this can be ignored unless you owned many shares of the PTP - your income for the state - other than your resident state is probably below the filing threshold for most states. if required to file state returns you could be dealing with 15 states or more.
