UPDATE: I found out that this does not seem to be happening if I use the original data feed and only one resampled data feed like so:
# convert the dataframe to a data feed, the timeframe of the original data feed must be set
data = bt.feeds.PandasData(dataname=dataframe, timeframe=bt.TimeFrame.Minutes, compression=1, fromdate=fromdate, todate=todate, plot=True)
# add the original data feed to cerebro
cerebro.adddata(data)
# add upsampled data feeds
cerebro.resampledata(data, timeframe=bt.TimeFrame.Days, compression=1)
If I run this, I don't get the duplicated timestamps as shown above. However, another thing that I noticed is that the close of a resampled bar will only be available during the next bar of the fast data feed (in the case above the minute data feed). Is this expected behavior?