There is a mistake in the code.
I'm not sure why the compiler does not report an error:
Code:

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action enemy_fight // attached to the enemy
{
while(plBiped01_entity==null){wait(1);}
   	   distance = vec_dist (my.x, plBiped01_entity.x); 	
   	   enemy = me; // I'm the enemy	
   	   enemy.healthpoints = 100; // and I have 100 healthpoints	 
   	   while(!plBiped01_entity) { wait(1); }  // wait until the player exists	
   	   {

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Can you see it?
Do you see it now:
Code:

---

   	   while(!plBiped01_entity) { wait(1); }  // wait until the player exists	
   	   {

---

It should be:
Code:

---

   	   while(!plBiped01_entity) { wait(1); }  // wait until the player exists	
           while(my.healthpoints > 0)
   	   {

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Furthermore you may replace these lines:
Code:

---

enemy = me; // I'm the enemy	
enemy.healthpoints = 100; // and I have 100 healthpoints

---

With this:
Code:

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my.healthpoints = 100;

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The enemy = my; stuff makes only sense if you have on enemy at a time.
But its useless if you use many enemies.