diff --git a/icu4c/source/common/uloc_tag.cpp b/icu4c/source/common/uloc_tag.cpp
index 0e1743699ce..1c10c48182c 100644
--- a/icu4c/source/common/uloc_tag.cpp
+++ b/icu4c/source/common/uloc_tag.cpp
@@ -1558,10 +1558,8 @@ _appendLDMLExtensionAsKeywords(const char* ldmlext, ExtensionListEntry** appendT
                 return;
             }
 
-            if (!_addAttributeToList(&attrFirst, attr)) {
-                *status = U_ILLEGAL_ARGUMENT_ERROR;
-                return;
-            }
+            // duplicate attribute is ignored, causes no error.
+            _addAttributeToList(&attrFirst, attr);
 
             /* next tag */
             pTag += len;
diff --git a/icu4c/source/test/cintltst/cloctst.c b/icu4c/source/test/cintltst/cloctst.c
index 4809f5cb8e6..e8ddf76e840 100644
--- a/icu4c/source/test/cintltst/cloctst.c
+++ b/icu4c/source/test/cintltst/cloctst.c
@@ -255,8 +255,8 @@ void addLocaleTest(TestNode** root)
     TESTCASE(TestLikelySubtags);
     TESTCASE(TestToLanguageTag);
     TESTCASE(TestBug20132);
+    TESTCASE(TestBug20149);
     TESTCASE(TestForLanguageTag);
-    TESTCASE(TestInvalidLanguageTag);
     TESTCASE(TestLangAndRegionCanonicalize);
     TESTCASE(TestTrailingNull);
     TESTCASE(TestUnicodeDefines);
@@ -6200,35 +6200,6 @@ static void TestForLanguageTag(void) {
     }
 }
 
-/* See https://unicode-org.atlassian.net/browse/ICU-20149 .
- * Depending on the resolution of that bug, this test may have
- * to be revised.
- */
-static void TestInvalidLanguageTag(void) {
-    static const char* invalid_lang_tags[] = {
-        "zh-u-foo-foo-co-pinyin", /* duplicate attribute in U extension */
-        "zh-cmn-hans-u-foo-foo-co-pinyin", /* duplicate attribute in U extension */
-#if 0
-        /*
-         * These do not lead to an error. Instead, parsing stops at the 1st
-         * invalid subtag.
-         */
-        "de-DE-1901-1901", /* duplicate variant */
-        "en-a-bbb-a-ccc", /* duplicate extension */
-#endif
-        NULL
-    };
-    char locale[256];
-    for (const char** tag = invalid_lang_tags; *tag != NULL; tag++) {
-        UErrorCode status = U_ZERO_ERROR;
-        uloc_forLanguageTag(*tag, locale, sizeof(locale), NULL, &status);
-        if (status != U_ILLEGAL_ARGUMENT_ERROR) {
-            log_err("Error returned by uloc_forLanguageTag for input language tag [%s] : %s - expected error:  %s\n",
-                    *tag, u_errorName(status), u_errorName(U_ILLEGAL_ARGUMENT_ERROR));
-        }
-    }
-}
-
 static const struct {
     const char  *input;
     const char  *canonical;
@@ -6558,3 +6529,18 @@ static void TestBug20370() {
         log_err("FAIL: Expected LCID value of 0 for invalid localeID input.");
     }
 }
+
+// Test case for ICU-20149
+// Handle the duplicate U extension attribute
+static void TestBug20149() {
+    const char *localeID = "zh-u-foo-foo-co-pinyin";
+    char locale[256];
+    UErrorCode status = U_ZERO_ERROR;
+    int32_t parsedLen;
+    locale[0] = '\0';
+    uloc_forLanguageTag(localeID, locale, sizeof(locale), &parsedLen, &status);
+    if (U_FAILURE(status) ||
+        0 !=strcmp("zh@attribute=foo;collation=pinyin", locale)) {
+        log_err("ERROR: in uloc_forLanguageTag %s return %s\n", myErrorName(status), locale);
+    }
+}
diff --git a/icu4c/source/test/cintltst/cloctst.h b/icu4c/source/test/cintltst/cloctst.h
index f415dbbe278..a88a49cf8d8 100644
--- a/icu4c/source/test/cintltst/cloctst.h
+++ b/icu4c/source/test/cintltst/cloctst.h
@@ -124,7 +124,6 @@ static void TestLikelySubtags(void);
  * language tag
  */
 static void TestForLanguageTag(void);
-static void TestInvalidLanguageTag(void);
 static void TestToLanguageTag(void);
 static void TestBug20132(void);
 static void TestLangAndRegionCanonicalize(void);
@@ -133,6 +132,7 @@ static void TestToUnicodeLocaleKey(void);
 static void TestToLegacyKey(void);
 static void TestToUnicodeLocaleType(void);
 static void TestToLegacyType(void);
+static void TestBug20149(void);
 
 /**
  * locale data