Double negatives and the present perfect continuous: recent or ongoing activity?

1. Is it correct to use two nots in a sentence like: ‘Don’t dare not to do it’?

2. Does the present perfect continuous tense have two meanings? What makes me think that is the comparison of two sentences like: ‘It has been snowing’ and ‘I have been learning English for five years.'

double negatives

Whilst not very common, double negatives are fine in standard English, provided they both carry a full meaning, as in your example. Further examples might be:

• 'Never do nothing!' (I.e. Always do something!)
• 'Never say 'no', if he asks you to help him.' (I.e. Sometimes say 'yes'.)
• 'Don't think about not coming to the station to see me off. I shall be so disappointed if you're not there.'

In non-standard English, in certain dialects, two, or even three, negatives may be used to express a single negative meaning. It is important to recognise these dialectal forms, though it might sound strange if you used them actively yourself. Here are a few examples:

• 'He didn't do nothing!' (= He didn't do anything OR He did nothing.)
• 'Since I got home last night, I ain't spoke to nobody nowhere.' (= Since I arrived home last night, I haven't spoken to anybody anywhere.)
• 'We ain't got no beer left. Shall I get some?' (= We haven't got any beer left. Shall I fetch some?)