Norwegian law does not outright ban dual citizenship, but it is severely restricted. A person may obtain dual citizenship at birth (for example by being born in the United States by a mother with Norwegian citizenship). A foreign citizen who applies for Norwegian citizenship must renounce their foreign citizenship. Only if that is not possible may the person keep their foreign citizenship.

A Norwegian citizen who applies for foreign citizenship does not keep their Norwegian citizenship.

From what I am able to find on the web, Eva Joly became a French citizen in 1967. She married a French man that same year (in Asker in Norway, just outside of Oslo; not in France). If French law bestowed French citizenship on a person who marries at French national -- and this was compulsory, then she could keep her Norwegian citizenship. However, if she applied for French citizenship, her previous Norwegian citizenship would have been AUTOMATICALLY lost.

The text of of the relevant Norwegian legislation from 1951 to 2006 (the law is still the same, but the citizenship act was revised a few years ago), translated into English.

§ 7

Norwegian nationality is lost by
1. a person who acquires the nationality of another state upon application or by express consent,
[...]
http://www.legislationline.org/documents/action/popup/id/4670

In French:

Titre 2.
De la perte de nationalité norvegienne.

Article 7.

Perd la nationalité norvegienne, celui qui:
1. obtient la nationalité d'un autre pays suite à sa demande ou qui accorde con consentement exprès à ladite perte,
[...]

http://www.ub.uio.no/ujur/ulovdata/lov-19501208-003-fre.pdf

Text of the Norwegian act at the time:

Kapitel 2. Korleis folk misser borgarretten.

§ 7. Norsk borgarrett misser:
1. den som får borgarrett i eit anna land etter søknad eller samtykke med reine ord,

http://www.lovdata.no/oll/hl-19501208-003.html#7

The current Norwegian act has the same rule:
§ 23. Tap ved erverv av annet statsborgerskap

Den som erverver annet statsborgerskap etter søknad eller uttrykkelig samtykke, taper sitt norske statsborgerskap.
[...]
http://www.lovdata.no/all/hl-20050610-051.html#23

This is an report by shyla stylez:

Eva Joly, the ruthless fraud prosecutor who nailed the 1990s corruption scandal at oil company Elf, is to run for French president.

Joly stunned France by winning the primary race for France's new expanded environmental party, Greens-Europe-Ecology, beating the favourite, TV presenter Nicolas Hulot, with 58% of the vote against Hulot's 41%.

Her score was seen as a victory for her theory of "combative" environmentalism. She has promised to attack lobby groups and financial interests and suggested creating an international court for crimes against the environment. She is a fierce advocate of pulling France out of its dependency on nuclear power.

The maverick 67-year-old with trademark red glasses is a household name in France. Her pursuit of corruption at the highest reaches of the French elite inspired film director Claude Chabrol's dark thriller A Comedy of Power,