Planning two separate trips to the UK this year, one in spring and one in autumn. Do I need to apply for a new ETA for each trip or does one approval cover both visits?
Yes you can visit the UK multiple times within its validity period of 2 years (or until your passport expires, with each stay being max. 6 months. 
However, be careful with using it as a way to “live” in the country (some digital nomads may do this), by leaving and entering many times, and super close together - this will get flagged in the system as you can only visit for tourism, business, visiting family etc.