[docs] fix double 'the' typos across the codebase (#13176)

This commit fixes instances of "the the" typos found in various
files across the codebase, including documentation, headers, source
files, and test scripts.

tests/nexus/test_1_1_5_6_9.cpp

@@ -321,7 +321,7 @@ void Test5_6_9(void)
     Log("---------------------------------------------------------------------------------------");
     /**
      * Step 10: Router_1 (DUT)
-     * - Description: Depending on the the DUT’s implementation, automatically sends new stable network data to SED_1
+     * - Description: Depending on the DUT’s implementation, automatically sends new stable network data to SED_1
      *   via a unicast MLE Child Update Request or MLE Data Response.
      * - Pass Criteria: The DUT MUST send EITHER a unicast MLE Child Update Request OR a unicast MLE Data Response to
      *   SED_1, which includes the following TLVs:

tests/nexus/verify_1_1_5_6_9.py

@@ -199,7 +199,7 @@ def verify(pv):
         must_next()
 
     # Step 10: Router_1 (DUT)
-    # - Description: Depending on the the DUT’s implementation, automatically sends new stable network data to SED_1
+    # - Description: Depending on the DUT’s implementation, automatically sends new stable network data to SED_1
     #   via a unicast MLE Child Update Request or MLE Data Response.
     # - Pass Criteria: The DUT MUST send EITHER a unicast MLE Child Update Request OR a unicast MLE Data Response to
     #   SED_1, which includes the following TLVs:

tests/scripts/thread-cert/border_router/MATN/MATN_09_DefaultBRMulticastForwarding.py

@@ -111,7 +111,7 @@ class MATN_09_FailureOfPrimaryBBROutboundMulticast(thread_cert.TestCase):
         br1.interface_down()
         self.simulator.go(180)
 
-        # 4b. BR_2 detects the missing Primary BBR and becomes the the Leader of
+        # 4b. BR_2 detects the missing Primary BBR and becomes the Leader of
         # the Thread Network.
         self.assertEqual('disabled', br1.get_state())
         self.assertEqual('leader', br2.get_state())