One example is with ghostscript recipe, where it has several CVE patches in SRC_URI. It happen where bumping ghostscript source version to 9.21 has included one of the CVE patch, CVE-2016-7977.patch. While we are supposed to remove the redundant patch, but do_patch does not flag any patch error and it appears that the patch was being patched on the source again.

Checking on the ghostscript source within build/tmp/work, the code was patched:
----- snip -----
lib_file_open(gs_file_path_ptr  lib_path, const gs_memory_t *mem, i_ctx_t *i_ctx_p,
                       const char *fname, uint flen, char *buffer, int blen, uint *pclen, ref *pfile)
{   /* i_ctx_p is NULL running arg (@) files.
     * lib_path and mem are never NULL
     */
    bool starting_arg_file = (i_ctx_p == NULL) ? true : i_ctx_p->starting_arg_file;
    bool search_with_no_combine = false;
    bool search_with_combine = false;
    char fmode[2] = { 'r', 0};
    gx_io_device *iodev = iodev_default(mem);
    gs_main_instance *minst = get_minst_from_memory(mem);
    int code;

>>>>>    if (i_ctx_p && starting_arg_file)
>>>>>        i_ctx_p->starting_arg_file = false;

>>>>>    if (i_ctx_p && starting_arg_file)
>>>>>        i_ctx_p->starting_arg_file = false;

    /* when starting arg files (@ files) iodev_default is not yet set */
    if (iodev == 0)
        iodev = (gx_io_device *)gx_io_device_table[0];
----- snip -----

It was an coincidence that the duplicated code does not have impact to compilation, but this is still an issue.

---

Meanwhile, in devtool, the patch was identified as "applied".

ERROR: Applying 'CVE-2016-7977.patch' failed:
checking file psi/zfile.c
Reversed (or previously applied) patch detected!  Assume -R? [n] 
Apply anyway? [n] 
Skipping patch.
1 out of 1 hunk ignored
ERROR: Function failed: patch_do_patch
ERROR: Logfile of failure stored in: /data/rebeccas/sdk-installer/poky_sdk/tmp/work/i586-poky-linux/ghostscript/9.21-r0/devtooltmp-jv777rue/temp/log.do_patch.43041
NOTE: Tasks Summary: Attempted 3 tasks of which 0 didn't need to be rerun and 1 failed.
ERROR: Extracting source for ghostscript failed