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Can it be possible that a word can at one and the same time mean something and its opposite? Yes it can. 

Specialists call it an enantioseme. Probably the example quoted most often in French is un hôte, meaning both a host and his guest. To avoid ambiguity, the latter can be called un invité. 

To indicate to friends in a restaurant that you intend to foot the bill, you might say: Vous êtes mes/nos invité(e)s. Curiously, une hôtesse does not involve the same ambivalence: she welcomes people to her home or to an exhibition, or as une hôtesse de l’air (whose male counterpart is not un hôte but un steward) looks after passengers on a flight. 

There are plenty more: the poet Charles Baudelaire felt it necessary to distinguish un crépuscule du matin from un crépuscule du soir. But there are also other everyday occurrences: un écran can both allow you to see and prevent you from seeing something; remercier is both to thank and to dismiss someone. 

Indeed, the simple word merci can lead to misunderstanding. Your host offers you a second helping of a delicious but cholesterol-charged dessert: Vous voulez encore du dessert? You say merci, even merci beaucoup, intending to decline the offer, conscious of your waistline. An extra serving lands on your plate! 

What went wrong? Your answer was not made with a firm falling tone, accompanied by a gesture declining the offer or by the little word non. For it is often necessary to say merci oui or merci non to make your intention absolutely clear. 

As it is, you make the most of the second helping and thank your host profusely with as clear a conscience as you can muster.