tl;dr: The error in the system was fixed.
synopsis: multiple coins may have the same symbol, and coins are viewed by symbol, but new coins are stored by id. Because 'later' coin ids in the CoinMarketCap-index overwrite 'earlier' ones, I didn't have a good grasp of coins I had already captured, causing the SQL error when I tried to reintroduce a 'new' coin (with the same symbol name) that I already had in the data-store.
Now, my index of coins include all coin-ids of the coin-symbols I have.
Recommendation system is back up and running.