Update the ClaimsIdentity/ClaimsPrincipal.GetClaim() extension to throw an exception when multiple claims of the same type exist

src/OpenIddict.Abstractions/OpenIddictResources.resx

@@ -1575,6 +1575,9 @@ To apply post-logout redirection responses, create a class implementing 'IOpenId
   <data name="ID0422" xml:space="preserve">
     <value>A configuration manager must be attached to the client registration to be able to resolve the server configuration.</value>
   </data>
+  <data name="ID0423" xml:space="preserve">
+    <value>Multiple claims of the same type are present in the identity or principal.</value>
+  </data>
   <data name="ID2000" xml:space="preserve">
     <value>The security token is missing.</value>
   </data>

src/OpenIddict.Abstractions/Primitives/OpenIddictExtensions.cs

@@ -1462,10 +1462,13 @@ public static class OpenIddictExtensions
     }
 
     /// <summary>
-    /// Gets the claim value corresponding to the given type.
+    /// Gets the unique claim value corresponding to the given type.
     /// </summary>
     /// <param name="identity">The identity.</param>
     /// <param name="type">The type associated with the claim.</param>
+    /// <exception cref="InvalidOperationException">
+    /// Multiple claims using the same <paramref name="type"/> are present in the identity.
+    /// </exception>
     /// <returns>The claim value.</returns>
     public static string? GetClaim(this ClaimsIdentity identity, string type)
     {
@@ -1479,14 +1482,43 @@ public static class OpenIddictExtensions
             throw new ArgumentException(SR.GetResourceString(SR.ID0184), nameof(type));
         }
 
-        return identity.FindFirst(type)?.Value;
+        var claims = identity.FindAll(type);
+        if (claims is IList<Claim> list)
+        {
+            return list switch
+            {
+                [Claim claim] => claim.Value,
+                [           ] => null,
+                [     ..    ] => throw new InvalidOperationException(SR.GetResourceString(SR.ID0423))
+            };
+        }
+
+        else
+        {
+            using var enumerator = claims.GetEnumerator();
+            while (enumerator.MoveNext())
+            {
+                var claim = enumerator.Current;
+                if (!enumerator.MoveNext())
+                {
+                    return claim.Value;
+                }
+
+                throw new InvalidOperationException(SR.GetResourceString(SR.ID0423));
+            }
+
+            return null;
+        }
     }
 
     /// <summary>
-    /// Gets the claim value corresponding to the given type.
+    /// Gets the unique claim value corresponding to the given type.
     /// </summary>
     /// <param name="principal">The principal.</param>
     /// <param name="type">The type associated with the claim.</param>
+    /// <exception cref="InvalidOperationException">
+    /// Multiple claims using the same <paramref name="type"/> are present in the principal.
+    /// </exception>
     /// <returns>The claim value.</returns>
     public static string? GetClaim(this ClaimsPrincipal principal, string type)
     {
@@ -1500,11 +1532,37 @@ public static class OpenIddictExtensions
             throw new ArgumentException(SR.GetResourceString(SR.ID0184), nameof(type));
         }
 
-        return principal.FindFirst(type)?.Value;
+        var claims = principal.FindAll(type);
+        if (claims is IList<Claim> list)
+        {
+            return list switch
+            {
+                [Claim claim] => claim.Value,
+                [           ] => null,
+                [     ..    ] => throw new InvalidOperationException(SR.GetResourceString(SR.ID0423))
+            };
+        }
+
+        else
+        {
+            using var enumerator = claims.GetEnumerator();
+            while (enumerator.MoveNext())
+            {
+                var claim = enumerator.Current;
+                if (!enumerator.MoveNext())
+                {
+                    return claim.Value;
+                }
+
+                throw new InvalidOperationException(SR.GetResourceString(SR.ID0423));
+            }
+
+            return null;
+        }
     }
 
     /// <summary>
-    /// Gets the claim values corresponding to the given type.
+    /// Gets all the claim values corresponding to the given type.
     /// </summary>
     /// <param name="identity">The claims identity.</param>
     /// <param name="type">The type associated with the claims.</param>
@@ -1525,7 +1583,7 @@ public static class OpenIddictExtensions
     }
 
     /// <summary>
-    /// Gets the claim values corresponding to the given type.
+    /// Gets all the claim values corresponding to the given type.
     /// </summary>
     /// <param name="principal">The claims principal.</param>
     /// <param name="type">The type associated with the claims.</param>

test/OpenIddict.Abstractions.Tests/Primitives/OpenIddictExtensionsTests.cs

@@ -2412,6 +2412,40 @@ public class OpenIddictExtensionsTests
         Assert.Equal("principal", exception.ParamName);
     }
 
+    [Fact]
+    public void ClaimsIdentity_GetClaim_ThrowsAnExceptionForDuplicateClaimType()
+    {
+        // Arrange
+        var identity = new ClaimsIdentity();
+        identity.AddClaim(Claims.Name, "Bob le Bricoleur");
+        identity.AddClaim(Claims.Name, "Bob le Bricoleur");
+
+        // Act and assert
+        var exception = Assert.Throws<InvalidOperationException>(() =>
+        {
+            identity.GetClaim(Claims.Name);
+        });
+
+        Assert.Equal(SR.GetResourceString(SR.ID0423), exception.Message);
+    }
+
+    [Fact]
+    public void ClaimsPrincipal_GetClaim_ThrowsAnExceptionForDuplicateClaimType()
+    {
+        // Arrange
+        var principal = new ClaimsPrincipal(new ClaimsIdentity());
+        principal.AddClaim(Claims.Name, "Bob le Bricoleur");
+        principal.AddClaim(Claims.Name, "Bob le Bricoleur");
+
+        // Act and assert
+        var exception = Assert.Throws<InvalidOperationException>(() =>
+        {
+            principal.GetClaim(Claims.Name);
+        });
+
+        Assert.Equal(SR.GetResourceString(SR.ID0423), exception.Message);
+    }
+
     [Fact]
     public void ClaimsIdentity_GetClaim_ReturnsNullForMissingClaims()
     {