bug: compile_commands.json may have duplicate data

[HansolChoe]

compile_commands.json may have duplicate data(not entirely duplicated because command is slightly different, but compilation target file is same) like below:

[
    {
        "directory": "/home/workspace/src",
        "command": "gcc -D HAVE_CONFIG_H -I /home/workspace/src -g -O2 -std=gnu99 -Wall -Wextra -Wdeclaration-after-statement -Wpointer-arith -funsigned-char -D _FORTIFY_SOURCE=2 -Wcast-align -Wcast-qual -Wshadow -Wbad-function-cast -Wwrite-strings -Wundef -Wuninitialized -Winit-self -Wnested-externs -Wstrict-prototypes -Wmissing-prototypes -Wmissing-declarations -Waggregate-return -pipe -MT /home/workspace/src/g72x.lo -MD -MP -MF .deps/g72x.Tpo -c -fPIC -D PIC -o /home/workspace/src/.libs/g72x.o -I /usr/lib/gcc/x86_64-linux-gnu/7/include -I /usr/local/include -I /usr/lib/gcc/x86_64-linux-gnu/7/include-fixed -I /usr/include/x86_64-linux-gnu -I /usr/include /home/workspace/src/g72x.c",
        "file": "/home/workspace/src/g72x.c"
    },
   ...
    {
        "directory": "/home/workspace/src",
        "command": "gcc -D HAVE_CONFIG_H -I /home/workspace/src -g -O2 -std=gnu99 -Wall -Wextra -Wdeclaration-after-statement -Wpointer-arith -funsigned-char -D _FORTIFY_SOURCE=2 -Wcast-align -Wcast-qual -Wshadow -Wbad-function-cast -Wwrite-strings -Wundef -Wuninitialized -Winit-self -Wnested-externs -Wstrict-prototypes -Wmissing-prototypes -Wmissing-declarations -Waggregate-return -pipe -MT /home/workspace/src/g72x.lo -MD -MP -MF .deps/g72x.Tpo -c -o /home/workspace/src/g72x.o -I /usr/lib/gcc/x86_64-linux-gnu/7/include -I /usr/local/include -I /usr/lib/gcc/x86_64-linux-gnu/7/include-fixed -I /usr/include/x86_64-linux-gnu -I /usr/include /home/workspace/src/g72x.c",
        "file": "/home/workspace/src/g72x.c"
    },
  ...
]

The command below does not have -fPIC -D PIC but other data are same.
This is confusing.

This symptom can be reproduced by cxbuild captured command at root of this file(fixed.1.zip). It is normal that .xdb/artifacts.zip is empty.