In SwiftKey you have the option to enable 'Quick period', which means that double-tapping the spacebar will insert a period followed by a space.
If you have selected the 'Auto insert prediction' setting, then repeatedly hitting the spacebar will continue to insert the middle prediction.
You can edit these settings in the following way:
- Open the SwiftKey settings
- Tap 'Typing'
- Tap 'Typing & Autocorrect'
- Toggle the 'Quick period' setting on or off