When you define a calculated field, you can specify the name of the field that the eval expression will create or modify. If the name of the calculated field matches the name of an existing extracted field, the calculated field will override the extracted field and replace its value with the result of the eval expression. This means that the original value of the extracted field will not be available for searching or analysis. To avoid this, you should use a unique name for your calculated field or use a different name for your extracted field2
1: Splunk Core Certified Power User Track, page 9. 2: Splunk Documentation, Configure calculated fields with props.conf.