Early arrival flight ?? visa office open at airport ??

I can't advise you on the opening hours of the visa office, but past reports have said that arriving passengers have sometimes had to go hunting for someone to open the office and provide the inbound visa service.

Note that it seems you now need visa support (a letter of invitation) to get a visa on arrival:

Notes: Please, be informed that for processing of visa (except private visa) in Consulate Bureau of Consular Department of MFA in the airport of Dushanbe for the period which is less than 45 days foreign citizens should submit filled visa support or visa application (2 copies), 2 photos and invitation. If foreign citizen fail to submit above mentioned documents, visa processing and issuance will not be produced and these persons will be deported from the Republic of Tajikistan.

(http://mfa.tj/index.php?node=article&id=455)

Many flight arrive after midnight. There is usually someone there, or someone who can run and get the consular official. The sole exception I can recall during our three years there was during one or another Eid, when it took hours and hours.

thanks heaps, really helpful
Barbara