Nested CMake projects multiple definition error when adding header-only library - c++

Question

I have a C++ library organized like:
Library/
CMakeLists.txt
main.cpp
src/
file1.cpp
include/
file1.h
Domains/
CMakeLists.txt
BlackJack/
CMakeLists.txt
src/
foo1.cpp
include/
foo1.h
...
libs/
tapkee/
CMakeLists.txt
include/
foo.hpp
....
Each library(i.e. Domains) contains linking and simple code for organizational purposes:
project(Domains)
cmake_minimum_required(VERSION 2.8)
add_subdirectory(IDomain)
add_subdirectory(Blackjack)
add_library(Domains STATIC)
target_link_libraries(Domains IDomain BLACKJACK)
Each sub-library (i.e. Blackjack) has a really simple CMakeLists.txt that just compiles the library (add_library).
This has been working just fine, until now.
I'm adding a third-party library (http://tapkee.lisitsyn.me/). This is a header-only library. It looks like CMake doesn't support this, but I found some work-arounds. I create a empty dummy.cpp file. Here is the CMakeLists for the third-party library:
project (TAPKEE)
file(GLOB TAPKEE_INCLUDE
"include/tapkee/*.hpp"
"include/tapkee/callbacks/*.hpp"
"include/tapkee/defines/*.hpp"
"include/tapkee/external/*.hpp"
"include/tapkee/neighbors/*.hpp"
"include/tapkee/parameters/*.hpp"
"include/tapkee/routines/*.hpp"
"include/tapkee/traits/*.hpp"
"include/tapkee/utils/*.hpp"
)
add_library(${PROJECT_NAME} header_only_BS.cpp ${TAPKEE_INCLUDE})
Here is the weird thing: My main.cpp and anything in the root src directory can include this tapkee library just fine, but none of the sub-libraries can (i.e. BlackJack). I get thousands of redefinition linker errors for the functions within tapkee.
Here is an example of the top-level CMakeLists:
project(ManifoldLearning)
#Root
set(SRC_DIR ${CMAKE_CURRENT_SOURCE_DIR}/src)
set(INCLUDE_DIR ${CMAKE_CURRENT_SOURCE_DIR}/include)
#Domains
set(BLACKJACK_INCLUDE_DIR ${CMAKE_CURRENT_SOURCE_DIR}/Domains/Blackjack/include)
set(TAPKEE_INCLUDE_DIR ${CMAKE_CURRENT_SOURCE_DIR}/libs/tapkee/include/)
include_directories(${INCLUDE_DIR} ${SRC_DIR})
include_directories(${TAPKEE_INCLUDE_DIR})
include_directories(${BLACKJACK_INCLUDE_DIR})
add_subdirectory(Domains)
#add_subdirectory(libs/tapkee)
add_executable(${PROJECT_NAME} ${SRC_LIST} ${HEADER_LIST})
target_link_libraries(${PROJECT_NAME} Domains)
I've been stuck on this for days, and any help/advice would be appreciated.
Update: I've removed the inclusion of the tapkee CMakeLists.txt (add_subdirectory) as per Antonio's direction. I still get redefinition errors, which I find strange. It's now not compiled anywhere and only included in BlackJack/src/foo1.cpp