is it bad to charge your iphone twice a day? But I've only done it once

Hi

Apple advises that you can charge your iPhone whenever you like. There is no need to let the battery discharge to any particular level first, nor any benefit to be gained from doing so:

https://www.apple.com/batteries/why-lithium-ion/

Apple's tips for maximising battery performance and battery lifespan are available here:

https://www.apple.com/batteries/maximizing-performance/

When charging your iPhone, be sure to follow all of Apple's safety and handling guidelines, including those detailed here:

• http://help.apple.com/iphone/10/#/iph63eecc618
• Keeping iPhone, iPad, and iPod touch within acceptable operating temperatures - Apple Support
• http://help.apple.com/iphone/10/#/iphbbe12ba1
• https://www.apple.com/batteries/maximizing-performance/

They include using only a genuine Apple Lightning to USB Cable or an MFi-certified third-party cable. Apple also recommends using the Apple USB Power Adapter that came with your iPhone (or another compatible Apple power adapter).

You can charge your phone as often as you like. Clearly, the battery life will be expended more quickly. Read this apple document to understand.

https://www.apple.com/uk/batteries/