diff --git a/GleeBug/Debugger.Thread.cpp b/GleeBug/Debugger.Thread.cpp
index f05d445..e68db0b 100644
--- a/GleeBug/Debugger.Thread.cpp
+++ b/GleeBug/Debugger.Thread.cpp
@@ -76,6 +76,16 @@ namespace GleeBug
     void ThreadInfo::StepInto(const StepCallback & cbStep)
     {
         StepInto();
+        
+        auto target = cbStep.target<void()>();
+        for (const auto & cb : stepCallbacks)
+        {
+            if (target == cb.target<void()>())
+            {
+                puts("duplicate StepInto callback detected!");
+                return;
+            }
+        }
         stepCallbacks.push_back(cbStep);
     }
 
diff --git a/GleeBug/GleeBug.h b/GleeBug/GleeBug.h
index 6cbd9c6..c4faa4e 100644
--- a/GleeBug/GleeBug.h
+++ b/GleeBug/GleeBug.h
@@ -5,6 +5,7 @@
 #include <string>
 #include <vector>
 #include <unordered_map>
+#include <unordered_set>
 #include <map>
 #include <set>
 #include <functional>