Value passed to function doubling inside call - c++

Question

I'm currently having an issue with a function call of get_set() for a doubly-linked list class I am making. The function returns a vector of all the elements within the list from position_from to position_to. For the tests I am sending
dll_UT->get_set( dll_UT->size()/2+1 , dll_UT->size()-1)
however, the value of dll_UT->size() somehow doubles within the call. When called directly before in my COUT statement, it shows the correct value of 100, but when inside the function this turns to 200. I've left console logs below as well as screenshots of the test.cpp and doubly_linked_list.cpp.
Any help on this is greatly appreciated as I've never seen this type of behavior before and would like to understand why such a thing is occuring.
Doubly_linked_list get_set(unsigned position_from, unsigned_position to){
std::cout << "\n\t__NEW TEST__\nSize of size/2+1 = " << this->size()/2+1 << "\nSize of size()-1 = " << this->size()-1 << std::endl;
std::cout << "position_from = " << position_from << "\nposition_to = " << position_to << std::endl;
//value of position_from == 200, should be == this->size()/2+1 which = 100
std::vector<int> temp;
temp.reserve(position_to - position_from);
while(position_from < position_to){
temp.push_back( this->get_data(position_from) );
position_from++;
}
temp.push_back(this->get_data(position_to));
std::cout << "Size of position_from = " << position_from << "\nSize of position_to = " << position_to << "\n\t__END TEST__" << std::endl;
return temp;```
}
Test.cpp {
```std::cout << "\nTEST.cpp::: dll_UT-size() = " << dll_UT->size() <<
"\nTEST.cpp::: dll_UT-size()/2+1 = " << dll_UT->size()/2+1 << std::endl;
//assuring dll_UT->size()/2+1 = 100
std::cout << "\nTEST.cpp::: expected_set.size() = " << expected_set.size() <<
"\nTEST.cpp::: expected_set.begin()+expected_set.size()/2+1 = " << expected_set.size()/2+1 << std::endl;
ASSERT_EQ(std::vector<int> (expected_set.begin()+expected_set.size()/2+1,expected_set.end()),
dll_UT->get_set( dll_UT->size()/2+1 , dll_UT->size()-1));//second half of data
// ^first operand doubled, ^second operand fine
//actually value sent is 200. dll_UT->size() is somehow doubling and equaling 400 but only for first operand of get_set(unsigned, unsigned)
Updated Caputre