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 If LorenTz ""derived the formula independently in 1870"", he was obviously a bright lad!  (As the linked Wikipedia article shows) he was only born in 1853. 

 Lorenz and his work are often confused with that of the more famous H A Lorentz.  In this case, it *might* be a doubled attribution of a single derivation.  Note the closeness of dates, 1869 and 1870.  An alternative suggestion is that Lorenz (the older man, without a 't') first *publicized* the formula in 1869 (e.g. at a physics conference), and had it published the following year (e.g. in a journal).  Further research could yeasay or naysay this. 

 (The article may be derived from from ""The Hutchinson Encyclopedia; September 22, 2003; 191 Words   ... the mathematical relationship between the refractive index and the density of a medium was published by Lorenz in 1869 and by Hendrick Lorentz (who discovered it independently) in 1870 and is therefore called the Lorentz-Lorenz formula . Using his electromagnetic ..."") 
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