Disable duplicate declaration validation in Acorn

I’m using Acorn to parse some syntactically valid JavaScript code into an ESTree for further processing. It appears that Acorn does some semantic checks too – in particular it throws an error for duplicate declarations. For example, parsing the following code throws an error of Identifier ‘f’ has already been declared: I do not want such semantic errors to be checked – I’m doing custom processing on the resultant ESTree, so the semantic validity of the source code does not matter to me. I’ve looked though the Acorn options for the parse(input, options) function, but I could not find anything