private string GetUniqueStringForModel<T>(string originalString, IEnumerable<T> enumerableObjects, Func<T, string> stringFieldFunction) where T : class
{
    var uniqueString = originalString;
    var duplicateCount = 1;

    while (enumerableObjects.Select(stringFieldFunction).ToList().Any(currentString => string.Equals(currentString, uniqueString, StringComparison.InvariantCultureIgnoreCase)))
    {
        uniqueString = originalString + "_" + duplicateCount++;
    }

    return uniqueString;
}

// ...
var list = enumerableObjects.Select(stringFieldFunction).ToList();
while (list.Any(currentString => string.Equals(currentString, uniqueString, StringComparison.InvariantCultureIgnoreCase)))
{
    uniqueString = originalString + "_" + duplicateCount++;
}