Identical compiler warnings in problems panel

### Brief Issue Summary
When compiling a project with multiple translation units that have warnings in some common include file they are shown multiple times. All of them work and open the same file in the same position.

### Expected:

There are no identical warnings.

### Apparent Behavior:

Identical warnings are shown.

Minimal project to reproduce:
CmakeLists.txt:
```cmake
cmake_minimum_required(VERSION 3.16)
project(Test)

add_executable(Test main.cpp Foo.cpp)
target_compile_options(Test PRIVATE -Wextra)
```
main.cpp:
```cpp
#include "Foo.hpp"
#include "Duplicate.hpp"

int main() { return 0; }
```
Foo.hpp:
```cpp
#pragma once

#include "Duplicate.hpp"
```
Foo.cpp:
```cpp
#include "Foo.hpp"
```
Duplicate.hpp:
```cpp
#pragma once

class Duplicate {
public:
void test(int i) {}
};
```
Multiple unused-parameter warnings should be shown in problems pannel.

Build output:
```
[main] Building folder: Test clean
[build] Starting build
[proc] Executing command: /usr/bin/cmake --build /mnt/media/dev/projects/Test/build/Clang-linux/Debug --config Debug --target clean --
[build] [1/1 100% :: 0.003] Cleaning all built files...
[build] Cleaning... 3 files.
[build] Build finished with exit code 0
[main] Building folder: Test
[build] Starting build
[proc] Executing command: /usr/bin/cmake --build /mnt/media/dev/projects/Test/build/Clang-linux/Debug --config Debug --target all -- -j 18
[build] [2/3 33% :: 0.022] Building CXX object CMakeFiles/Test.dir/Foo.cpp.o
[build] In file included from ../../../Foo.cpp:1:
[build] In file included from ../../../Foo.hpp:3:
[build] ../../../Duplicate.hpp:5:17: warning: unused parameter 'i' [-Wunused-parameter]
[build] void test(int i) {}
[build] ^
[build] 1 warning generated.
[build] [2/3 66% :: 0.022] Building CXX object CMakeFiles/Test.dir/main.cpp.o
[build] In file included from ../../../main.cpp:1:
[build] In file included from ../../../Foo.hpp:3:
[build] ../../../Duplicate.hpp:5:17: warning: unused parameter 'i' [-Wunused-parameter]
[build] void test(int i) {}
[build] ^
[build] 1 warning generated.
[build] [3/3 100% :: 0.330] Linking CXX executable Test
[build] Build finished with exit code 0
```

### Platform and Versions

- **Operating System**: Arch Linux
- **CMake Version**: 3.17.3
- **VSCode Version**: Latest insiders
- **CMake Tools Extension Version**: 1.4.1
- **Compiler/Toolchain**: Clang 10.0.0

### Other Notes/Information

Reproduce the issue with the provided CMakeLists.txt and main.cpp, Foo.cpp, Foo.hpp, and Duplicate.hpp files using the shown Clang build. Start from how compiler warnings are surfaced in the problems panel and compare entries from the two translation units. Done means identical warnings appear only once while retaining the same file and position navigation.

Written by the indexing model from the issue text.