Added test case when field was already shot (expect false)

src/main/java/de/tims/fleetstorm/matchfield/Coordinate.java

@@ -45,6 +45,9 @@ public class Coordinate {
 	}
 
 	public boolean shoot() {
+		if (this.state == Coordinate.SHOT)
+			return false;
+
 		if (this.state == Coordinate.SHIP) {
 			this.state = Coordinate.HIT;
 			return true;

src/test/java/de/tims/fleetstorm/matchfield/CoordinateTest.java

@@ -1,6 +1,7 @@
 package de.tims.fleetstorm.matchfield;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertTrue;
 import static org.junit.jupiter.api.Assertions.assertEquals;
 
@@ -74,6 +75,7 @@ class CoordinateTest {
 		matchfield.setShip(new Coordinate(3, 3), 4, 1);
 
 		assertTrue(matchfield.getField(0, 0).shoot());
+		assertFalse(matchfield.getField(0, 0).shoot());
 	}
 
 }