mysql – Delete from GROUP BY with multiple GROUP columns

I have a simple table consisting of id, channel_id, value and timestamp. The combination of channel_id,timestamp MUST be unique but the table has gotten corrupted. My ultimate goal is to add a unique index which fails due to the duplicates.

The duplicates are:

select channel_id,timestamp,count(*) cnt 
from data 
group by channel_id,timestamp having cnt>1

How can I delete the duplicates? I’m stuck since obviously I can’t get the row id from the duplicates sub select?

UPDATE I could probably do

delete d1.* from data d1 join data d2 on d1.channel_id=d2.channel_id and d1.timestamp=d2.timestamp where !=;

but since the table has almost 1 billion records a cross-product does not seem like a good idea?