Extract from ACADISO.LIN states
;; User Defined Linetypes
;; Add any linetypes that you define to this section of
;; the file to ensure that they migrate properly when
;; upgrading to a future AutoCAD version. If duplicate
;; linetype definitions are found in this file, items
;; in the User Defined Linetypes section take precedence
;; over definitions that appear earlier in the file.
This is not true in 2013.
If you define a linetype e.g. Center in this section of the file
and this has the same name as a linetype that appears previously in the file,
using the command sequence
Linetype Load will display both linetypes in the dialogue.
and if you use in a macro e.g.
(command "Linetype" "Load" "Center" "Acadiso.lin" "")
Autocad loads the first linetype named Center it finds in the file
and then tries to load the next one, but with a
" reload linetype" prompt..