+= to sensor rather then keep adding the value of 2 sensors

Configuration
1

Hi all.

I have 2 history stats sensors and one template sensor. The template sensor should hold the value of the 2 history stats sensors. It works but instead of having the value of both sensors, it seems to keep adding on top of the previous value causing the time spent in the shower to show as double to what it really is.

# First and Second Bathroom Shower On Time
  - sensor:
      - name: "First and Second Bathroom Shower On Time"
        unique_id: First and Second Bathroom Shower On Time
        state: >-
          {{ states('sensor.first_bathroom_shower_on_time') | float(0) + states('sensor.second_bathroom_shower_on_time') | float(0)}}