Logistic for duplicates as a function of covariates (fast)
Source:R/logisticdupbyx_fast.R
      logisticdupbyx_fast.RdAs logisticdupbyx, but faster when distance is a covariate
(but no interactions with distance occur.
R/logisticdupbyx_fast.R
      logisticdupbyx_fast.RdAs logisticdupbyx, but faster when distance is a covariate
(but no interactions with distance occur.