Something worth considering for next year is this Twitter conversation:
Querying HTTPArchive it seems the problem is real. 2.2% of sites (155k sites!) request the EXACT SAME font in WOFF and WOFF2 format 
However, often WOFF2 fails so tries WOFF after. So no double download, but still bad for perf.
1% of sites do a full double download of both.
| mobile(abs) | desktop(abs) | mobile(%) | desktop(%) | |
|---|---|---|---|---|
| Downloads same font as WOFF and WOFF2 | 155,541 | 132,044 | 2.17% | 2.19% |
| Downloads same font as WOFF and WOFF2(both return status 200) | 69,805 | 59,387 | 0.97% | 0.99% |
Data and Queries: WOFF and WOFF2 double downloads - Google Sheets
