Kim B. Heino writes:

When installing a new package with dpkg dependencies are checked 
correctly. But when I try to update an existing package, the 
dependencies are checked against the old package, not against new 
package. Thus the new package can break dependencies.

archival/dpkg.c

@@ -643,11 +643,9 @@ static unsigned fill_package_struct(char *control_buffer)
 		return -1;
 	}
 	num = search_package_hashtable(new_node->name, new_node->version, VER_EQUAL);
-	if (package_hashtable[num] == NULL) {
-		package_hashtable[num] = new_node;
-	} else {
-		free_package(new_node);
-	}
+	if (package_hashtable[num] != NULL)
+		free_package(package_hashtable[num]);
+	package_hashtable[num] = new_node;
 	return num;
 }