Lookahead(cheating) issue

[rightx2]

duplicated counting issue

Bug/Feature Request Description

I will quote my stack overflow question and answer: https://stackoverflow.com/a/61261300/3595632.
The problem is that feature generated by dfs() count the same value that cutoff_time Dataframe already count: When Jeff calculating lt, he already counted the amount of "2012-03-01" of store 0 for the aggregated amount of cutoff_time = "2012-03-01"(store 0) (and I think this makes sense). So, 1 month amount aggregation period: 2012-03-01 ~ 2012-03-31, March, 2012.
Let's see SUM(sales.amount) column value of (store_id = 0 & 2012-03-01 cutoff time). The value is 44 by adding up the amounts of 2012-02-05, 2012-02-10, and """2012-03-01"""!!!. It counts """2012-03-01""" here again, which is a sort of cheating and look-ahead bias.

2012-03-01 is March, not February. We must predict 1 month total amount of March, using ONLY February information....

Expected Output

I think that SUM(sales.amount) column value of (store_id = 0 & 2012-03-01 cutoff time) must be 16 + 15 = 31, excluding 2012-03-01 amount.