Where to start troubleshooting with "duplicate symbol"?

Question

I'm learning C++ on Mac and trying to follow along with the SFML Game Development by Example book. Any obvious places to start looking for problems when I get:

duplicate symbol __Z2dpi in:
    /var/folders/f1/4ddx4sk171v4hp6s50c93zh40000gn/T/sfgamewindow-374355.o
    /var/folders/f1/4ddx4sk171v4hp6s50c93zh40000gn/T/statemanager-3f5acc.o
[ plus 34 more similar duplicate symbols]

? Would that have anything to do with #including?

This is my make command:

cd ~/Programming/CPP;
clang++  -g -O0 -Wall -std=c++14 \
-o Testgame         \
-I. -I/Users/johnrebekah/Programming/SFML\ source/include \
-I/Users/johnrebekah/Programming/CPP/gameheaders \
-lsfml-window.2.5.1     \
-lsfml-system.2.5.1     \
-lsfml-graphics.2.5.1   \
 "/Users/johnrebekah/Programming/CPP/gameheaders/sfgamewindow.cpp" \
 "/Users/johnrebekah/Programming/CPP/gameheaders/statemanager.cpp" \
 "/Users/johnrebekah/Programming/CPP/gameheaders/introstate.cpp" \
 "/Users/johnrebekah/Programming/CPP/gameheaders/eventmanager.cpp" \
 "/Users/johnrebekah/Programming/CPP/gameheaders/game.cpp" \
 "/Users/johnrebekah/Programming/CPP/testgame.cpp" \

All of the listed duplicate symbols are from the first file in the list of cpps in the make command, paired with various other of the cpp files...