Comment 15 for bug 1843415

Moreover, the check seems not to include the master sheet name, so you can create a subsheet with the same name of the top schematic. (see attached file as an example project created using master branch, on Linux, with "casesensitive.sch and caseSensitive.sch).
Trying to create sub-sub-sheets gives you the pop-up warning.

Application: Eeschema
Version: (5.99.0-373-gf8aea249d-dirty), release build
Libraries:
    wxWidgets 3.0.2
    libcurl/7.52.1 GnuTLS/3.5.8 zlib/1.2.8 libidn2/0.16 libpsl/0.17.0 (+libidn2/0.16) libssh2/1.7.0 nghttp2/1.18.1 librtmp/2.3
Platform: Linux 4.9.0-8-amd64 x86_64, 64 bit, Little endian, wxGTK
Build Info:
    Build date: Nov 16 2019 07:30:35
    wxWidgets: 3.0.2 (wchar_t,wx containers,compatible with 2.8) GTK+ 2.24
    Boost: 1.69.0
    OpenCASCADE Community Edition: 6.8.0
    Curl: 7.52.1
    Compiler: GCC 6.3.0 with C++ ABI 1010

Build settings:
    KICAD_SCRIPTING=ON
    KICAD_SCRIPTING_MODULES=ON
    KICAD_SCRIPTING_PYTHON3=OFF
    KICAD_SCRIPTING_WXPYTHON=ON
    KICAD_SCRIPTING_WXPYTHON_PHOENIX=OFF
    KICAD_SCRIPTING_ACTION_MENU=ON
    BUILD_GITHUB_PLUGIN=ON
    KICAD_USE_OCE=ON
    KICAD_USE_OCC=OFF
    KICAD_SPICE=ON