Why is ddiod spelt with two d's here? When we are taught it earlier in the lesson it only has one d.
Do you mean as in "Diod". If so "Diod" is the pre-mutated form. In some situations it can mutate softly to "ddiod". It has mutated here because it comes after the person (i).
yes sadly the course seems to have avoided including a section where they teach mutations.
The notes that refer to these grammar points don't seem to be available in the mobile version.
There are some notes at the beginning of the lesson that introduce soft mutation. I've found it helpful to read the lesson notes before getting started.
Why was it gaf in the last sentence and ga here? In both it was may I have a drink.
No, mutations only affect the first letters of words or when two words form a compound word. Here "Ga" is a more modern form of "Gaf".
"I have" is never [in my 60yr plus experience of English in the UK, US and Europe] contracted to "I've" in questions.
This is a problem with the Duo system at the moment. The course teams can do nothing about it.
It would be helpful if you could spare a little time to report it on the general 'Duolingo troubleshooting discussion' (search the web for that term to find it quickly) - there is likely to be an existing thread to which you can add your example of the error. That discussion is monitored by Duo staff.