i dont have the yet the code for finding the duplicate lines and arcs.. still finding one in the web..
If you want to write your own function you could use the same approach as what is used in OVERKILL that resolves overlapping lines. See the acet-overkill-resolve-lines2 and acet-overkill-resolve-arcs2 functions in overkillsup.lsp.
If you intend to only use the lisp code in your office I'd recommend using OVERKILL rather than writing a new routine. OVERKILL isn't perfect, but it has been available for a long time and it works fairly well. OVERKILL.LSP is just an interface. All the code to do the cleanup is in OVERKILL.SUP. It is easy to link directly to the functions in OVERKILLSUP.LSP.
Martin