mathematics - Should I include independently derived but not novel results in my thesis?

I'm writing up my physics PhD thesis, and am wondering whether to include the derivation of a formula that I worked out myself. I've recently found that this work isn't actually a novel contribution to the field, as the formula (though not a detailed derivation) appears in the appendix of a 10 year old paper.

Is there any sense in including it? And, if so, how would one describe the contribution?