According to Errata item 2.106:

2.106. Section 13.6. State that textStandardDecimalSeparator,
textStandardGroupingSeparator, textStandardExponentRep, textStandardInfinityRep, textStandardNanRep and textStandardZeroRep must all be entirely distinct from one another, and it is a schema definition otherwise.

I created a test where the grouping separator matched the decimal separator (both "."), and the parse fails with an ambiguous parse error:

[error] Test edu.illinois.ncsa.daffodil.section13.text_number_props.TestTextNumberProps.test_textStandardGroupingSeparator03 failed: edu.illinois.ncsa.daffodil.processors.ParseError: Parse Error: Convert to Double (for xs:double): Unable to parse '5.000.000.00' (using up all characters).

See test textStandardGroupingSeparator03 in the following files:
daffodil-test/src/test/resources/edu/illinois/ncsa/daffodil/section13/text_number_props/TextNumberProps.tdm
daffodil-test/src/test/resources/edu/illinois/ncsa/daffodil/section13/text_number_props/TextNumberProps.tdml
daffodil-test/src/test/scala-debug/edu/illinois/ncsa/daffodil/section13/text_number_props/TestTextNumberPropsDebug.scala