Duplicate DW_AT_abstract_origin not pruned when "attribute" integrates

[pmachata]

$ ./dwgrep/dwgrep ./libzwerg/libzwerg.so.0.1 -e '
    entry (name == "overload_op") ?DW_TAG_GNU_call_site attribute'
low_pc (addr)   0x60ed7;
abstract_origin (ref4)  [3680b6];
sibling (ref4)  [36f1e5];
abstract_origin (ref4)  [368084];
linkage_name (strp) _ZN11overload_opC2ESt10shared_ptrI2opE17overload_instance;
high_pc (data8) 33;
frame_base (exprloc)    0..0xffffffffffffffff:[0:call_frame_cfa];
object_pointer (ref4)   [3680da];
GNU_all_call_sites (flag_present)   true;
specification (ref4)    [34c2b1];
inline (data1)  DW_INL_not_inlined;
external (flag_present) true;
name (strp) overload_op;
decl_file (data1)   /home/petr/proj/dwgrep-older/libzwerg/overload.cc;
decl_line (data1)   205;
accessibility (data1)   DW_ACCESS_public;