diff --git a/core/common/src/format/DateTimeFormatBuilder.kt b/core/common/src/format/DateTimeFormatBuilder.kt
index a235b7de..2230051d 100644
--- a/core/common/src/format/DateTimeFormatBuilder.kt
+++ b/core/common/src/format/DateTimeFormatBuilder.kt
@@ -130,6 +130,7 @@ public sealed interface DateTimeFormatBuilder {
          * [am] is used for the AM marker (0-11 hours), [pm] is used for the PM marker (12-23 hours).
          *
          * Empty strings can not be used as markers.
+         * [IllegalArgumentException] is thrown if either [am] or [pm] is empty or if they are equal.
          *
          * @see [amPmHour]
          */
diff --git a/core/common/src/format/LocalDateFormat.kt b/core/common/src/format/LocalDateFormat.kt
index 165f9ac2..21484c70 100644
--- a/core/common/src/format/LocalDateFormat.kt
+++ b/core/common/src/format/LocalDateFormat.kt
@@ -13,7 +13,7 @@ import kotlinx.datetime.internal.format.parser.Copyable
 /**
  * A description of how month names are formatted.
  *
- * An empty string can not be a month name.
+ * An [IllegalArgumentException] will be thrown if some month name is empty or there are duplicate names.
  */
 public class MonthNames(
     /**
@@ -23,7 +23,14 @@ public class MonthNames(
 ) {
     init {
         require(names.size == 12) { "Month names must contain exactly 12 elements" }
-        names.forEach { require(it.isNotEmpty()) { "A month name can not be empty" } }
+        names.indices.forEach { ix ->
+            require(names[ix].isNotEmpty()) { "A month name can not be empty" }
+            for (ix2 in 0 until ix) {
+                require(names[ix] != names[ix2]) {
+                    "Month names must be unique, but '${names[ix]}' was repeated"
+                }
+            }
+        }
     }
 
     /**
@@ -67,7 +74,7 @@ internal fun MonthNames.toKotlinCode(): String = when (this.names) {
 /**
  * A description of how day of week names are formatted.
  *
- * An empty string can not be a day-of-week name.
+ * An [IllegalArgumentException] will be thrown if some day-of-week name is empty or there are duplicate names.
  */
 public class DayOfWeekNames(
     /**
@@ -77,7 +84,14 @@ public class DayOfWeekNames(
 ) {
     init {
         require(names.size == 7) { "Day of week names must contain exactly 7 elements" }
-        names.forEach { require(it.isNotEmpty()) { "A month name can not be empty" } }
+        names.indices.forEach { ix ->
+            require(names[ix].isNotEmpty()) { "A day-of-week name can not be empty" }
+            for (ix2 in 0 until ix) {
+                require(names[ix] != names[ix2]) {
+                    "Day-of-week names must be unique, but '${names[ix]}' was repeated"
+                }
+            }
+        }
     }
 
     /**
diff --git a/core/common/src/internal/format/parser/ParserOperation.kt b/core/common/src/internal/format/parser/ParserOperation.kt
index 02ba4c76..6ac3a1c2 100644
--- a/core/common/src/internal/format/parser/ParserOperation.kt
+++ b/core/common/src/internal/format/parser/ParserOperation.kt
@@ -167,6 +167,7 @@ internal class StringSetParserOperation<Output>(
                     node.children[searchResult].second
                 }
             }
+            require(!node.isTerminal) { "The string '$string' was passed several times" }
             node.isTerminal = true
         }
         fun reduceTrie(trie: TrieNode) {
diff --git a/core/common/test/format/LocalDateFormatTest.kt b/core/common/test/format/LocalDateFormatTest.kt
index 88758b99..d1363253 100644
--- a/core/common/test/format/LocalDateFormatTest.kt
+++ b/core/common/test/format/LocalDateFormatTest.kt
@@ -221,6 +221,38 @@ class LocalDateFormatTest {
         assertEquals("2020 Jan 05", format.format(LocalDate(2020, 1, 5)))
     }
 
+    @Test
+    fun testEmptyMonthNames() {
+        val names = MonthNames.ENGLISH_FULL.names
+        for (i in 0 until 12) {
+            val newNames = (0 until 12).map { if (it == i) "" else names[it] }
+            assertFailsWith<IllegalArgumentException> { MonthNames(newNames) }
+        }
+    }
+
+    @Test
+    fun testEmptyDayOfWeekNames() {
+        val names = DayOfWeekNames.ENGLISH_FULL.names
+        for (i in 0 until 7) {
+            val newNames = (0 until 7).map { if (it == i) "" else names[it] }
+            assertFailsWith<IllegalArgumentException> { DayOfWeekNames(newNames) }
+        }
+    }
+
+    @Test
+    fun testIdenticalMonthNames() {
+        assertFailsWith<IllegalArgumentException> {
+            MonthNames("Jan", "Jan", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec")
+        }
+    }
+
+    @Test
+    fun testIdenticalDayOfWeekNames() {
+        assertFailsWith<IllegalArgumentException> {
+            DayOfWeekNames("Mon", "Tue", "Tue", "Thu", "Fri", "Sat", "Sun")
+        }
+    }
+
     private fun test(strings: Map<LocalDate, Pair<String, Set<String>>>, format: DateTimeFormat<LocalDate>) {
         for ((date, stringsForDate) in strings) {
             val (canonicalString, otherStrings) = stringsForDate
diff --git a/core/common/test/format/LocalTimeFormatTest.kt b/core/common/test/format/LocalTimeFormatTest.kt
index 595bcc52..80934fc3 100644
--- a/core/common/test/format/LocalTimeFormatTest.kt
+++ b/core/common/test/format/LocalTimeFormatTest.kt
@@ -216,6 +216,24 @@ class LocalTimeFormatTest {
         }
     }
 
+    @Test
+    fun testEmptyAmPmMarkers() {
+        assertFailsWith<IllegalArgumentException> {
+            LocalTime.Format {
+                amPmMarker("", "pm")
+            }
+        }
+    }
+
+    @Test
+    fun testIdenticalAmPmMarkers() {
+        assertFailsWith<IllegalArgumentException> {
+            LocalTime.Format {
+                amPmMarker("pm", "pm")
+            }
+        }
+    }
+
     private fun test(strings: Map<LocalTime, Pair<String, Set<String>>>, format: DateTimeFormat<LocalTime>) {
         for ((date, stringsForDate) in strings) {
             val (canonicalString, otherStrings) = stringsForDate