Heat effects are applied twice. -31% In your speed penalties results in an overall speed of 38.
Heat effects should be applied on a body-part by body-part basis. Since things like Left leg and Right leg are separate body parts, they will indeed get double the heat effects in many cases.
The bug isn’t that it’s applied on a limb by limb case. If you total the penalties listed in the debuff section and it = 31. you will lose 62 speed in game even though it only displays a penalty of 31.