If 2 lamps of same power are connected in series, the first one connected glows better and the next one gets dimmer..then Why doesnt the serial lights( Christmas lights ) get dimmer even though they are connected in series and share the same voltage..??
Christmas lights don't exactly "share" the same voltage on each lamp - they happen to have approximately the same voltage but that is because they are all in series and have roughly the same impedance to current when glowing.
If your supply is 240V ac and you have 20 lamps in series, each one will have approximately 12V across the lamp terminals. It makes sense for each lamp therefore to be rated for 12V operation and of course if you tried to put that across 240V it would instantly fail and blow.
If you took a 240V lamp and put it in series with another 240V lamp, each lamp would be receiving 120V and therefore the brightness would approximately reduce by a factor of four.
The answer to your second question is related to your first. If 2 lamps rated for the same voltage and power are connected in series, then the same current must flow through each one. Since they are rated for the same voltage and power, the resistance of their filaments must be the same. Thus if the current through them is the same, the voltage will also be the same. Hence, the power, which is the product of voltage and current, will also be the same. Thus they will glow equally. The same is true of a string of Christmas lights. It doesn't matter how many; they will still glow equally.
As was suggested in a comment by @JarrodChristman on the question, if the two lights claim to be rated the same, but they are not the same brightness when connected in series, then the lights do not actually have the same specs. Perhaps the filament on one bulb was extruded a little thinner than the other; even though they are both approximately the same rating, one has a slightly higher resistance, and burns a little hotter (and brighter, and will burn out sooner).
In a strand of Christmas lights, each bulb dissipates so little power (1A/125V on a 50 light string is ~2.5W per bulb) that a 5% manufacturing variance on the filaments means the bulbs are only going to vary by ±0.125W. With a 40W household bulb, a 5% manufacturing variance means the bulbs could vary by as much as ±2W.
I'd bet that on that string of Christmas lights you consider to be "equal" brightness, if you were to measure the luminance of each bulb individually (with total isolation from all other light sources) with a precise instrument, you would indeed find that some are not quite as bright as others.