[CMake] import/export targets

[Micha Renner]

In a top-level CMakeLists I have the following lines

PROJECT(BuildDLL14)
 
CMAKE_MINIMUM_REQUIRED(VERSION 2.6)
SET(CMAKE_MODULE_PATH "/usr/local/share/CMake")
SET(CMAKE_INSTALL_PREFIX ${CMAKE_BINARY_DIR})

ADD_SUBDIRECTORY(DLL-1)
ADD_SUBDIRECTORY(TestDLL)
-------------------------------------------
The CMakeLists.txt of DLL-1 looks like this:

PROJECT(DLL-1)
CMAKE_MINIMUM_REQUIRED(VERSION 2.6)
SET(_targetname SLib1)

INCLUDE_DIRECTORIES(${CMAKE_SOURCE_DIR}/DLL-2)

SET(_src cmFile.c)
SET(_imIncludeFiles cmFile.h)
 
ADD_LIBRARY(${_targetname} STATIC ${_src} ${_imIncludeFiles})
INSTALL(TARGETS ${_targetname} DESTINATION lib EXPORT lib1-targets)
INSTALL(EXPORT lib1-targets DESTINATION lib)

The CMakeLists.txt of TestDLL looks like this:
-------------------------------------------
PROJECT(TestDLL14)
 
CMAKE_MINIMUM_REQUIRED(VERSION 2.6)

INCLUDE(${CMAKE_INSTALL_PREFIX}/lib/lib1-targets.cmake)

INCLUDE_DIRECTORIES(${CMAKE_SOURCE_DIR}/DLL-1)

SET(_src MRTestC.c)  
ADD_EXECUTABLE(TestDll14 ${_src})
TARGET_LINK_LIBRARIES(TestDll14 _Lib1)

... and the result is:
-------------------------------------------
CMake Error at CMake/lib/lib1-targets.cmake:16 (ADD_LIBRARY):
add_library cannot create imported target "SLib1" because another target
  with the same name already exists.
Call Stack (most recent call first):
  TestDLL/CMakeLists.txt:10 (INCLUDE)
-- Configuring incomplete, errors occurred!

If one out-comment the ADD_SUBDIRECTORY(DLL-1) then it works.

With best regards

Michael